only in it for a good time

If you are asking what the expression means, you should say so, because we don't read minds around here.

To be "only in it for a good time" means that someone only does something in order to entertain him- or herself, and not for any of the other reasons people might have for doing that same thing. So, for example, a man might see a woman he considers pretty involved in a political demonstration. So he joins the demonstration, so that he can be near that woman. He doesn't care about the political issue, he's just in it for a good time.

if his other post, in a thread about soldiers and cheating, he claims to be a soldier in texas, who's in love with his wife but is open to some extra marital fun (email included)

as hard as it might be to believe, i'm skeptical

And here i assumed this might be a real human being . . . silly me . . .