Couldn't a same-sex couple adopt, just like a childless heterosexual couple, and commit themselves to each other exclusively, permanently, and unconditionally? In such a case, would it be fair to exclude them from marriage?

Even if it were possible for homosexuals to commit themselves to each other in the ways described, their relationships would still lack the orientation to procreation, the openness to life, that marriage is all about. This of itself means that any unions between homosexuals are not marriages, regardless of what people may wish to call them.