He's fantasizing about women he will never be with. Seems fairly harmless to me. Even if he's perfectly happy with your mom, there's no reason he wouldn't ALSO look at porn. What happens in your mind isn't a betrayal until you resolve to put it into action (as in, committing an act of cheating with another person). I don't think you need to worry about your step-dad and your mom's relationship just yet.
As for the other thing... You know, being uncomfortable around him... There's not much you can do about this but get over it, unless you've forged a close relationship with this guy. If you have (and he's the sensitive type you can talk to), why not just discuss it openly with him? Say it makes you feel uncomfortable, and say why (or that you don't know why, if you don't know the exact reason). Even if nothing gets resolved, maybe the whole situation will feel less awkward. Discuss things like adults if it's becoming a problem for you, or else just get over it. After all, it's easy to point the finger and not see the four pointing back at us, isn't it? Harmless fantasy of a sexual nature is something practically all men and women indulge in, to my knowledge.
Your gender and age (approximate age if you're uncomfortable with disclosing information online) might be useful to know regarding how to proceed. I guess I just realized in retrospect that this isn't a situation where most young girls would want to "talk it out" with the guy due to it being kind of uncomfortable, but it still seems like your best option (aside from forgetting about it completely, which I recommend, because in my opinion it really is nothing).