I will say this. In addition to the things Enzo said, which I agree with, i've always found the "love is love" argument to be utterly ridiculous. In that case, we should consider beastiality, necrophilia, and incest okay as well. Just on the bottom line of it. Not to mention, this "equality" bill business seems to be a soulless money/benefits grab than anything. As well as to say someone is born gay, or bisexual, or anything as an excuse, is too laughable to even comment on. (Just in case that gets brought up.) But anyway, the legality, again, as Enzo said, is pretty much pointless in the gays pursuit of "equality", since it doesn't change much besides them wanting some extra benefits. And since they cannot hold a true family is extremely selfish on their end. And as Enzo said, just be happy their group is the apple of this media, and entire country's eye.
One thing I agree with KB on is on their part, I don't understand why the officials changed their mind. That's just stupid whether I agree with it or not.
The only difference with your argument is that Necrophelia, Beastiality, Pedophelia & the like actually do cause harm to others and/or are more about pure sexual ecstasy, rather than two people actually having consensual sexual relations and sharing in a relationship. If you honestly believe that by giving gays the right to marry that it's instantly going to open a gateway for the harmful practices above, you don't understand the real legal purpose of the situation.