Oh, yes, the entire distinction comes down to which positions a couple decides to explore in the bedroom.
Without even getting into the radical psychological and emotional differences between mixed-sex and same-sex relationships, the reproductive aspect, alone, forms the unbridgeable chasm of separation.
Yet the rabidly pro-gay marriage crowd refuses to acknowledge there is a profound and fundamental difference between a relationship that can, biologically, though with exceptions, produce offspring, and one that cannot, under any circumstances whatsoever, mix its genetic materials and reproduce.
"Since some heterosexuals are sterile or barren - or choose not to have children - the reproductive capabilities of gays and straights is exactly the same! Case closed!" *stamp foot*