I'm willing to forgive David Bowie for nearly anything.

So I think I just figured out the motivation behind this tactic which wasn't obvious to me before (maybe it was to you). I doubt straight men innately dislike kissing or showing affection toward men. It seems more likely to me that they (okay, we) are either homophobic or wary of the status cost of being seen as gay or bisexual. Thus a straight male who declares himself to be bisexual demonstrates a rejection of homophobia and in part shows that he doesn't think being gay or bisexual is low status and refuses to accept some (but not all) of the privileges he has as a straight male (the privilege language is obviously controversial but it probably isn't to the people who do this).

The problem is part of the anti-gay narrative is that homosexuality isn't actually an important part of anyone's identity, that it isn't innate but basically just people choosing to be "sinful". Identifying as bisexual for political reasons bolsters this position. "If these straight males can choose to behave like bisexuals, then the bisexuals can choose to behave like good, church-going straight people!" Also, the fact is a straight male really can't take on the same persecution non-heterosexuals face. They can always opt out and they are never told that a part of their identity is immoral (their told that the act their putting on is immoral, but that isn't the same thing). And of course in some circles being gay or bisexual is a status booster- my friends would be suspicious I was "coming out" for these status-benefits, not out of a genuine attempt at solidarity. Actually, I've seen this complain leveled at some college-aged bisexual women.