Not natural, eh? You've flown, have you not? Perhaps you've traveled at a speed above around 20 mph? You might be living in a house, built from synthetic materials. You're almost certainly on a computer made by *gasp* unnatural materials. You think what is natural should determine what humans should and shouldn't do? Give up planes, trains, cars, computers and your house and I may listen to you.
The whole institution of marriage is fucking unnatural, why should loving somebody some people consider unnatural be denied from joining in this whole fucking charade?