My wife is the hinge in our V, with another male partner who is transitioning to female via estrogen therapy.
My wife wanted us to have a threesome together, so we tried.
It was a very odd experience.
For me, it's just too much like cuckoldry now. I don't want to know - I don't want to discuss it with them - etc. It's not the transgender thing at all. Such things just aren't my cup of tea.
Probably everyone would be on a spectrum when it comes to this, is my guess. I don't think it's a taboo topic.