I am bemused by the idea that just because everyone is gay that they would all be attracted to each other!!
Also when you rely on sexual attraction for your definition of familial harmony you will end up with the type of situation BG describes.
However I quite favour group marriage but not this orgy type of nonsense, but people who are united in friendship and commitment to the family, people can (and would, of course, being human) have various sexual romantic interest in any of the other people in the marriage. Such a large group would only work IMHO if people do not want and expect to have to sleep with anyone in the marriage they want to, some of the bonds would be friendship, some with benefits and some with greater or lesser romantic interest and jealousy tackled in just the same way as we work through it in any Poly relationship.
This whole idea that jealousy will be less of an issue because everyone is sexually attracted to each other is dangerously close to 'We want a Unicorn because then no one would be left out so there would be no jealousy and everyone would be happy' nonsense.