Avi K
1. we don’t nesserely pasken like the rambam for hilchos ben noach, the Rambam doesn’t include cannibalism but many achronim say it’s asur for a ben noach, as implied by a few chazals
2. Even the Rambam would assur lesbian “marriages” this is implist in the rambam’s understanding of marriage, which is one of the reasons the rambam felt the need to include in a halacha sefer the halacha before matan Torah on marriage despite it’s apparent irrelevance today. I would use the rambams in the beginning of hilchus ishus to assur it according to the rambam
3. the chazal quoted by the rambam in isurai biah makes it very clear that lesbiainsim is wrong for goyim. somethings that are not assur to a ben noach could sometimes be worse. The Torah mocks Lot even though al pi halacha the rambam paskens he did nothing wrong. We see that God even matured an issur for kayin in order to avoid a worse action by adam. The radak writes on ohr lagoyim that it’s to get the goyim to keep the 7 mitzvos and follow a derech yashar, there is no way that one can ever say that lesbianism much less marriage is a derech yashar!