I respond to it like I respond to any insult: I ignore it. Once someone has resorted to insults in an attempt to win an argument, it is immediately clear to anyone (or at least, anyone who's opinion is worth caring about) that you have won and there is simply no point in continuing to converse with this person.
I will say that I have a certain amount of respect for people who come up with content-appropriate insults instead of resorting to the (apparently) all-purpose Internet homophobia. I still won't talk to them, but I'll remember them as someone who may actually have a valid point and are simply having a difficult time explaining it. Frustration with your own inability to express yourself can result in an angry outburst.
I will say that I have a certain amount of respect for people who come up with content-appropriate insults instead of resorting to the (apparently) all-purpose Internet homophobia. I still won't talk to them, but I'll remember them as someone who may actually have a valid point and are simply having a difficult time explaining it. Frustration with your own inability to express yourself can result in an angry outburst.