Belz...
Fiend God
If consenting adults want to enter into a relationship that doesn't hurt anyone, they should be able to.
It's... distasteful for the overall society to basically go "Yeah you aren't hurting anyone, but we don't feel like dealing with the fallout so we're just going to invoke the vague 'harmful to society' to make it easier on us."
It's the same sort of logic used against gay marriage, anyway. Somehow it was going to cheapen real marriage or spread "the gay" to your children, etc. It was just common sense right until the point where it wasn't.