One argument, if it even deserves the term, against bisexuality, is that it would destroy the family and one on one relationships. The appropriate response is to ask this individual (if male): "Do you like redheads?" He will, most likely, answer yes. Then ask "Do you like blondes?" He will, again, probably answer yes. Then comes the coup de grace: "Do you require one of each?"
If he answers yes to that, tell him that he's against traditional values; if he answers no, you can say, "Neither do bisexuals." But he probably won't get it.
They never do, do they?