Well, sleeping with a woman doesn't necessarily make you straight or even bisexual. When looking at JP, for example, he grew up in a generation where (especially upper class people) were expected to at least appear heterosexual. As far as Matt's self-labeling, he goes with what he feels best suits him. He feels gay, so let him be gay. f**k labels. They're for other people's benefits. I could bang a chick if I really had to, but I'm gay! That's how I see myself. What someone else thinks of my sexuality is moot, because I think and feel gay.
I wouldn't mind an openly bi character in the CAP series. It's not a bad idea at all. I'm surprised their hasn't been at least one bisexual in the series, especially considering that the author is bisexual.