How could you know that "plenty of guys do that"? I've been through high school and if any guy I knew thought they were gay, they hid it very well... which is why I don't know how you know "plenty of guys" do it. I mean no offense, but if "plenty of guys" just means your own single experience, then don't try to convince the OP of your idea by telling him it's a common thing. But, of course, if you've done a scientific study, or can direct me to one, that shows it's so common, then I'm sorry for questioning you.
That said, I have known a handful of girls who, according to a mutual friend, told me they just acted bisixual because they didn't have any luck with guys. But I don't think that's particularly common for girls, or guys.
I meant it more as in; "People do it without even realizing it," kind of way. Plenty of people switch there playing fields because they think there having troubles with the six they should be attracted to; Same as your friends. Its mostly just mistaking things for what they aren't: And I'm sure people do that.
In the OPs sense; If the friend doesn't have a lot of friends, and isn't great with the ladies, then he might be mistaking the friendship for more then it is, because he isn't getting it anywhere else. But thats pure speculation on my part.
You have every right to question me; Mostly because its my opinion. But I'm sure this happens to a decent amount of people. Not openly, of course, but subtly.

The Lord of Death once thought he was gay!? With who? The Devil!?
He told me he just wanted to experiment.. :cold: