Equality is (supposedly) coming on leaps and bounds with all the minority groups in society being pushed to be more open and those in the majority pushed to accept them.
I believe marriage between any people, regardless of their gender, race, age and number should be legal. If a man loves a woman and that woman loves that man, who's to say that woman and that man can't also love another man who loves them, making a three-way relationship? And who's to say that a man who used to be a woman can't love a woman who used to be a man (see my favourite couple ever here)? If a group of people can legitimately find love, why may they not be joined?
That does open things up to family members, and whether they can love each other in a sexual way. I personally don't find this wrong, so long as any children are monitored for signs of genetic disease. This becomes even more broad when you look at this taken from Wikipedia, which provides evidence that even second-level relations are comparatively safer than first-level ones.
Children of parent-child or sibling-sibling unions are at increased risk compared to cousin-cousin unions












