Don't forget the neighbors two year old daughter or son.
If you have no moral compass anything goes.
Anticipating the gay marriage advocates, they'll tell you a child can't give consent, hence adult-child marriage isn't possible.
This problem does not obtain with multiple partners, animals (animals are property), or inanimate objects (property). Or cadavers, as they're also property.
The gay marriage advocates haven't a leg to stand on to prohibit (discriminate against) the outlandish "marriages" I mention, as they're no more outlandish than the one they advocate.
Besides, it really isn't about "marriage", it's about forcing other people to acknowledge their perversion as something to "celebrate", and to bring down the force of law on anybody who dares to call homosexuality for the undesirable perversion it is.