If I may relate from the perspective of a bi man:
I have a friend who I have known since 16, and honestly believe he is straight- his interest is in women and always had been. However, when he found that I was having sex with men as well as women I could see he had questions in his own mind. They "came out" recently when he was drunk- he quizzed me for 3/4 of an hour about what I liked about MM sex, did it hurt, did it wreck your arsehole, was anal sex messy, what was it like to suck a cock etc. I was frank in that it wasn't everyone's thing but many guys found it pleasurable and some rated it above sex with women. Not 2 days later, he invited me to his house and stone cold sober, asked me if I could introduce him to MM sex. I asked him to think hard about that one- our friendship would survive it, but what if he wants more?
In relation to the initial question- he would obviously have man sex with a bi friend and I guess for him it's that thing many curious guys find- you don't always know you aren't straight until you aren't straight.
I doubt I'll go there on principle, but I do have a bi mate who makes a specialty of providing curious guys with an unforgettable first time MM sex experience and I may introduce them on that basis. My bi mate swears that the only way to start off curious guys is on their back with legs spread, preceded by shower foreplay and rimming. I must say that his "return" rate is very high and at least one of these previously straight guys is now hooked on cock- in my regular personal experience.