Ugh I hate this topic. The OP is understandably confused, but a lot of the posts are simply dead wrong, and in fact, IMO dangerous.
Sexuality is not about who you fuck, it is about who you are. A person can't be turned gay by having a dick in his mouth or ass. That's stupid and frankly, insulting. If it is true then homophobia is not unreasoable at all. If a person can be turned gay by another gay man, then I could understand why people are homophobic.
Being gay is a mental process. It is an attraction to other men. Being gay is NOT about actually having sex with other men, though it is natural consequence of it. A man who is only attracted to men, but is so afraid of those feelings that he only has sex with women is not straight. He's gay. If you believe you become gay by having sex with other men, then he's straight, which obviously is not true.
The point here is that a man does not become gay after having sex with another man. If he wanted to have sex with him and had attractions to men, then he was gay beforehand. The sexual act didn't turn him gay.
To get back to the OP's question. Such people are most likely not straight. They are probably gay or bi, but are uncomfortable with that label, so they call themselves straight even though their sexual acts would indicate an attraction to men. I guess they may be in denial about it. But personally I am uncomfrtable about forcing an unwanted label upon them. I thught gay liberation was all about gaining the right not to have the "straight but mentally ill because he likes guys" label. If teh straight community doesn't have the right to impose that upon us, then we have no right to impose the "gay but is so afraid of it he's mentally ill" upon these guys.
Sexuality is a mental process and as such I am really uncomfortable telling people what they think, even if it may seem obvious from their actions. Thier minds are their own exclusive territory.