This is the only logical difficulty I see when I'm arguing gay rights with people. I've said in the past that there is no logical argument against gay marriage, and that's pretty much true, but looking deeper into the justification for gay marriage (people have the right to pursue happiness as long as it doesn't harm others or harm the functioning of society...and homosexuality does not harm anyone, whereas bestiality, pedophilia and incest are harmful to the animal, the child or the offspring).
The only problem is, how do you respond when your opponent points out that you must support incest as long as they don't have children?
The only problem is, how do you respond when your opponent points out that you must support incest as long as they don't have children?