“Is it OK for two adult siblings to form a gay couple? Should they be allowed to get married?
Is it OK for three people of the same sex who are insanely in love with each other to get married?
Why/why not?”

To me that is your own judgement. I find it troubling that you would equate being born “GAY” with being deviant. though I believe that if it makes them happy and they aren’t hurting anyone it’s their business not mine. Animals do all of the things your talking about. For me there is a choice there because I am not attracted to anyone in my family, the thought makes me neausious. Though some people are. If they aren’t hurting anyone you shouldn’t be able to stop them. Though their offspring may be affected by there behaviour. May be that’s why the government doesn’t allow it.