Personally, i think that if a guy is really serious about a gal,
he will not proposed something like the above....
but the thing is r u serious with him??
If you are, then u must re-consider ur r/s with him because it is
prob not mutual.
if u just play play in the r/s and u find the idea of 3-some
appealing... then maybe u can go ahead....
at least since it is ur bf and his frend, you will probably not be
physically harmed, unlike with some strangers or something.
I have actually contemplate 3-somes before with my ex... but cannot
find a suitable participant... so since u got a chance and all
along u have the desire, just go for it.