Just bouncing off another thread here at the moment.

In Ancient Rome, straight men commonly had sex (anal usually - oral was considered dirty) with other men and male slaves. In Ancient Rome, straight free men wished to be the penetrators rather than being the ones penetrated.

These days, men who identify as straight and who want to try some masculine energy are having "bro jobs". And they are cool with either penetrating or being penetrated.

It’s more of, like, you’re not supposed to do it kinda’ thing. Being straight, it just intrigues me. I don’t know. I like getting head and guys just seem to be better at it than girls because they have the package, I guess you could say. Another thing that intrigues me is the whole dick on dick thing. I don’t know how that got into my mind, why I like it. I can’t really explain it but something like that just turns me on.” That arousal is where John begins to question whether or not he is straight. When we first began talking he said most people would consider him bisexual; however, he has no desire to be in a relationship with another man. For him, that is what he says makes him straight.

http://www.huffingtonpost.com/albert-serna-jr/its-just-a-brojob-no-homo_b_9491738.html

Women might experiment with other women at a certain point in their lives, without being considered bi or gay. So what's different for men?

After a string of chats with men who seek out bro-jobs, there were two reasons I heard over and over. Firstly, sex with other men offers guys a no-strings-attached experience – a business deal of sorts. You don’t have to be a scientist to see why this is so appealing. In her research, Reynolds found these types of encounters often involve glory holes or blindfolds to keep the acts even more anonymous. And secondly, it allows men to take on a different role. With women, guys often feel they need to be the dominant one in the bedroom, but being with other men is all about giving in to their submissive side. Sex therapist Dr Susan Block adds “the BDSM element can involve humiliation by means of another penis, a larger penis”.

http://shesaid.com/au/meet-the-guys-who-identify-as-straight-but-have-sex-with-other-men/

Of course, it begs the question, what is straight and what is bisexuality and is there any use in making a distinction between them?