Hi "Dr." Bill,
Frankly, it sounds to me like this guy is one of these "bisexuals" who is basically gay but too ashamed to do much more than screw men on the sly while having a girlfriend to show to the world. These guys usually tell the men they get involved with that they like women, but rarely tell the women they are involved with that they like guys. [And gay men are perfectly capable of being good in bed with women; it's just not their preference.]I suggest you say to X exactly what you say in your question, if you haven't already.
But I have two thoughts [and keep in mind that I've been accused of being politically incorrect when it comes to the subject of bisexuality.] Bisexual men tend to be homosexual men who are ashamed of being gay and need women to a) make themselves feel more macho [along the lines of "real men fuck women"] and b) to provide a heterosexual front for themselves. Why give yourself this hassle? While it's great that you, as you say, find man-sex sexy and all that, why take a chance that your "bi" boyfriend will come to accept he's gay and/or develop romantic feelings for one of his playmates [whether he's gay or bi].
Lastly, I also have to say that there are a lot more straight men than gay men in the world, so why not leave the gay/bi guys alone for other men, LOL?