If we go by the stereotypical view that a 49 year old man leaves his 46 year old wife for his 26 year old secretary, his 46 year old wife might think, "Oh, okay, she's 20 years younger, she's got this, she's got that," and she can compare herself to another woman. Whereas if a 49 year old male lawyer, for example, is unable to resist a 26 year old gay male lawyer, his wife cannot compare herself to a man, and it would be expected that unlike in the first case, she could be thinking, "I don't know you at all."

Some couples might be able to stay together but have an unconventional agreement, in that, say the 49 year old man I alluded to above stays with his wife, but he goes and sees his male partner three nights a week or whatever.

If one member of a couple is bisexual, then, maybe they can have an open relationship.

Four things you cannot expect are: a heterosexual woman to become a lesbian because her husband comes out as a transwoman, a heterosexual man to come out as a homosexual because his wife comes out as a transman, a gay man to become a heterosexual because his partner comes out as a transwoman and a lesbian to become straight because her partner comes out as a transman. But one thing that couples should do, is especially if children are involved, not become a complete a-hole towards their ex-partner. I remember reading about a woman whose husband came out as gay (the one who found the packet of condoms in the boot of his company car, and who made calls to gay chatlines) who said that she would not allow their daughters to stay overnight at their father's place if he has a partner. Now, okay, if he was a drug dealer or drug user, and they went to his place to find drug paraphernalia and he had bloodshot eyes and the place stank of booze, smoke and vomit, I'd say understandable. But if her father has a partner and both are clean-living, I would say that a teenage girl would have no more risk, perhaps even less of one, than if her father was married to another woman. Why there might be less of a risk is that she might not, at 13 or 14, be left to babysit kids while her father goes out with his new partner. A stable same-sex relationship might be perfectly innocuous.