If she is cheating then she is not a lesbian at all but rather Bi-sexual.You need to have a long talk with her and establish whats going on.All the best.
Well, it's hard to say whether you really have something you should be worrying about, or whether your jealousy is just clouding your judgement. Jealousy can be a very destructive emotion.
Has your GF ever given you a reason to distrust you? Ever cheated? AS far as the texts and e-mails, did you used to always read those, or did you just start asking to see them as a result of your jealousy? I feel a person is entitled to their privacy, perhaps there are other reasons she doesn't really want you to be reading her messages? For example, if she's good friends with this guy, I'm sure she may do some venting to him about when you two maybe have a fight, that kind of thing. If that's the case, she probably doesn't want you reading those things, which is fair. On the other hand, if she isn't doing anything wrong and KNOWS you're worried, I would think she would just show them to you to ease your mind. So, I can see where that MAY be a bit of a red flag.
You seem very convinced this guy has a thing for your GF. How do you know that? Is your GF bi-sexual? I would think the fact that she's with YOU, a woman...would make it far less likely she would be cheating on you with a man, unless she is bi of course.
I think you need to communicate your feelings to your GF. If she isn't doing anything wrong, I would think she would put forth a good amount of effort to prove to you, and reassure you nothing is going on.
I think you should also post this thread in our "Relationships" community...there are some really smart posters down there who are pretty good at giving relationship advice.
Best of luck to you, hope it all works out!