Ok, so there are two possibilities here.
These guys are being legit sexist, and especially assholish sexist to boot. They are a bunch of total jerks and the only reason they shouldn't be pushed out into traffic is because it would traumatize the poor bus driver who ran them over.
On the other hand... It is possible that these guys are not being sexist assholes, but it is you and your girlfriend that are the problem. Let me explain.
In gaming groups there is always one person with more social power than all the rest. It might be because this is the person that buys the pizza, this is the person that owns the games, or this is the person who is the common friend (everyone is friends with this person, but not necessarily with everyone or anyone else.) This person has the most power in the group - they typically get the deciding vote on what is played, how, and when. They more or less run the game group. This is not a bad thing under normal circumstances. Everyone is in the game group because they are having fun with what is being done in the game group, and this natural leader is doing a good job of choosing games based on the group preference, he chooses good times for game night, etc. It all works out great.
So everything is fine. Except this person has a significant other. There is an old adage: love is blind. It is very difficult to accurately assess the faults of your significant other, especially from the perspective of someone else. The make the matter worse, this significant other has a great deal of sway over the defacto leader of the group. So, unless both the leader and the significant other are very careful the inevitable result is that the significant other exercises influence over the group to their own advantage, usually at a cost to the group. Example: Everyone wants to play civ V, but this significant other wants to play smash bros. So you play smash bros. Everyone wants to watch The Matrix, but this significant other wants to watch gladiator. So you watch gladiator. There is a conflict between the significant other and another person in the group and the leader gangs up against the other person in defense of their significant other.
And that is just what happens when this significant other exercises the influence they have over this person. Sometimes there are just basic personality conflicts and they are never going to get along. Sometimes the significant other is just an asshole and the leader just can't see it.
You are the defacto leader of this group because you made the club. Are you letting your girlfriend have influence over you? How often does she get her way? When there is a conflict with her and someone else do you always take her side?
Do you see how this can lead to resentment and outright hostility? They might like you just fine but see your girlfriend as this manipulative jerk that uses her influence over you to get her way all the time.
In my experience sexism in gaming groups manifests as condescension and inappropriate actions (creepin), not outright hostility ("she is such a *****.") I have never seen it manifest as a strait "girls should not be allowed" statement, ever, in my life, even when I was in junior high playing mario kart. Did they actually say, in plain words, that she should not be allowed because she is a girl? Or are you inferring it from their behavior towards what is probably the only girl in the club?
In addition, the pattern of an initial conflict, a period of peace, and then a gradual rise in hostility and resentment is very common in the second situation but not in the first. Sexism does not grow and spread over the short term. If a sexist person can be persuaded to shut up and stand having a girl in the group for a few weeks then it is unlikely that they will make a fuss about it later. The problem usually returns in the form of passive condescension against the woman.
But hostility and resentment will grow over time when there continues to be perceived unfairness. After the initial conflict people will tend to just hope the situation will get better and they shut up for a while. When it doesn't that is when the real hostility starts. In all probability you have already made it clear that you will take your girlfriends side in any and all conflict, which just makes it worse. What you perceive as sexism might be growing hostility and resentment due to your constant favoritism of your girlfriend over everyone else.
In either case, the immediate solution is the same: either kick out the offenders or leave the club or start a new one. If these guys are sexist assholes nothing you can do will change their behavior, and if the problem is you and your girlfriend then your best bet is finding people that can stand you two.
These guys are being legit sexist, and especially assholish sexist to boot. They are a bunch of total jerks and the only reason they shouldn't be pushed out into traffic is because it would traumatize the poor bus driver who ran them over.
On the other hand... It is possible that these guys are not being sexist assholes, but it is you and your girlfriend that are the problem. Let me explain.
In gaming groups there is always one person with more social power than all the rest. It might be because this is the person that buys the pizza, this is the person that owns the games, or this is the person who is the common friend (everyone is friends with this person, but not necessarily with everyone or anyone else.) This person has the most power in the group - they typically get the deciding vote on what is played, how, and when. They more or less run the game group. This is not a bad thing under normal circumstances. Everyone is in the game group because they are having fun with what is being done in the game group, and this natural leader is doing a good job of choosing games based on the group preference, he chooses good times for game night, etc. It all works out great.
So everything is fine. Except this person has a significant other. There is an old adage: love is blind. It is very difficult to accurately assess the faults of your significant other, especially from the perspective of someone else. The make the matter worse, this significant other has a great deal of sway over the defacto leader of the group. So, unless both the leader and the significant other are very careful the inevitable result is that the significant other exercises influence over the group to their own advantage, usually at a cost to the group. Example: Everyone wants to play civ V, but this significant other wants to play smash bros. So you play smash bros. Everyone wants to watch The Matrix, but this significant other wants to watch gladiator. So you watch gladiator. There is a conflict between the significant other and another person in the group and the leader gangs up against the other person in defense of their significant other.
And that is just what happens when this significant other exercises the influence they have over this person. Sometimes there are just basic personality conflicts and they are never going to get along. Sometimes the significant other is just an asshole and the leader just can't see it.
You are the defacto leader of this group because you made the club. Are you letting your girlfriend have influence over you? How often does she get her way? When there is a conflict with her and someone else do you always take her side?
Do you see how this can lead to resentment and outright hostility? They might like you just fine but see your girlfriend as this manipulative jerk that uses her influence over you to get her way all the time.
In my experience sexism in gaming groups manifests as condescension and inappropriate actions (creepin), not outright hostility ("she is such a *****.") I have never seen it manifest as a strait "girls should not be allowed" statement, ever, in my life, even when I was in junior high playing mario kart. Did they actually say, in plain words, that she should not be allowed because she is a girl? Or are you inferring it from their behavior towards what is probably the only girl in the club?
In addition, the pattern of an initial conflict, a period of peace, and then a gradual rise in hostility and resentment is very common in the second situation but not in the first. Sexism does not grow and spread over the short term. If a sexist person can be persuaded to shut up and stand having a girl in the group for a few weeks then it is unlikely that they will make a fuss about it later. The problem usually returns in the form of passive condescension against the woman.
But hostility and resentment will grow over time when there continues to be perceived unfairness. After the initial conflict people will tend to just hope the situation will get better and they shut up for a while. When it doesn't that is when the real hostility starts. In all probability you have already made it clear that you will take your girlfriends side in any and all conflict, which just makes it worse. What you perceive as sexism might be growing hostility and resentment due to your constant favoritism of your girlfriend over everyone else.
In either case, the immediate solution is the same: either kick out the offenders or leave the club or start a new one. If these guys are sexist assholes nothing you can do will change their behavior, and if the problem is you and your girlfriend then your best bet is finding people that can stand you two.