To address this question I'd first like to challenge your premise. I think you need to add stereotypically to the start, because we are not dealing with real individuals here are we? Of course when you do that, the question no longer has quite the same impact as it does if the stereotype is allowed to stand for actual bisexual men. But with this caveats in mind, to answer your question , it has probably got something to do with the fact that up until recently, it was a lot more problematic to be in a gay relationship. So if a guy feels a sexual attraction to another guy, but he also feels the weight of social prohibition and things that he could function in a heterosexual realationship, then he would be careful not to get too emotionally attached to the other guy. For otherwise he would have to cross the rubicon between heterosexuality and homosexuality. So emotional coldness is probably more about managing expectations than it is about an genuine emotional reserve.
