Silly axioms are usually included when someone says something isn't rational. Given modern human rights, there's no way to arrive at a no homo-marriage stance.
Your example statement isn't useful either, as plenty of same-sex couples are unable to create children. By the same logic, infertile people should also be excluded. If that was the point (marriage only for people seeking to reproduce), at least it'd be consistent.
Excluding poly-amorous people from marriage is a mean, anti-human thing, too.
>So are you saying if a CEO of a company believed euthanasia to be wrong you'd stop using that companies products?
It's more than just believing, it'd have to be that the CEO had performed actions to directly oppose euthanasia. And I generally don't boycott something unless I think using the product will directly contribute to hurting something and there's a reasonable alternative. For instance, buying a music CD directly gives money to a company that will lobby for DRM, against privacy, and for longer copyright. Whereas using FireFox is unlikely to directly help anti-gay campaigners. (I still hope there's backlash against FF and Eich, because screw his Prop8 stuff, and screw JS - but that's more of a "I just wanna see things burn" perspective.)