Heck no.  When did we decide that only gay men can enjoy their asses?  Men are lucky to have a prostate...the male counterpart to our g-spots.  Could you imagine someone telling you that you could only enjoy your g-spot if you were a lesbian?  Or that if a man gave you a g-spot orgasm you would somehow turn into a lesbian.  That would be silly, right?  Rest assure ladies, there is no reason to worry that somehow his sexual orientation has changed because of his newfound interest. Chances are he was very nervous even talking to you about it.  Now it's your job to support him and have fun exploring it for what it is...pleasure plain and simple!

That is not to say that there aren't bisexual men out there who enjoy a little assplay from time to time!