I'd say go for it, too. Unless any of the parties is someone the loss of whose friendship (e.g., a brother) would cause you lifelong ongoing harm. The risk of that happening is small, but real. (Sort of like the risk of a plane crashing...very unlikely to happen, but if it does, consequences horrible.)
In any even, though, I'd caution to be careful that you and your friend are fairly closely aligned in terms of your level of bisexuality, or lack thereof. If you're both on the same wavelength (whether it be, 'woah! no touching,' or 'man, I've always wanted to fool around with him/other guys").
Where you could get into trouble is if one but not both of you is markedly more (or less) interested in the M2M dynamic than the other.
If you don't know the guy well enough to talk about this, or if his sexual vibe (gay, straight, or in between) isn't crystal clear, then I'd say call it off or proceed with extreme caution.
Last thing you want to have happen is to lick the balls of an unsuspecting and unwilling friend, or to have to rebuff a vulnerable sexual overture from a good friend of yours toward you.
Good luck.