Why is homosexual relationships ok, but not incest relationships?

As the topic says.

Not trolling, currently in a discussion about lgbt and marriage, and they are using incest as a counter argument.

Pretty much every argument FOR homosexual marriage can also apply to incest relationships. The only real argument against incest relationships is that inbreeding results in a weaker gene pool, but they counter with 1) This inbreeding affect wont happen so quickly, 2) what if they simply dont have kids of their own and adopt like a homosexual relationship.

It's a tough point to counter argue, and I hope someone can give me some points.