I imagine different people define it in different ways.
It seems like an easy target for jokes. I have an image of a man and a mirror standing before a priest. Or someone explaining their relationship with themself, and what the terms of the relationship are. Then there’s the scene where the self-spouse tries to get into the swingers club as a couple.
I can imagine it being a serious thing for some people, and even making sense, though.
I think people should have the right to define who they are and what their contracts and relationships are.
Also some people get pretty confused. But such things may be part of their path to healing, and be a healthy thing.
I know of some people who marry inanimate objects. Or disembodied persons. Or gods.
There are some serious ancient spiritual traditions that include this. To name drop one that modern skeptics might know, the Virgin Mary is one. That wasn’t an original idea, by the way. There were many other spiritual women who had practices involving self conception and related ideas. Again, usually this was a spiritual metaphorical thing rather than the way modern “scientific” skeptics tend to relate to it (i.e. as literal woo woo super-powers that make no sense).