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AllosaurusJei That makes me wonder what Japanese law would make of something like shared motherhood IVF, i.e. where a lesbian couple has a child by having one partner donate their egg, and impregnating the other partner via IVF+a sperm donor. In that scenario, both the biological and birth mothers would have a right to the child. They couldn't invalidate the biological mother's parental rights for obvious reasons, and also would have a very difficult time invalidating the birth mother's rights, since there is plenty of precedent with straight women who used a donor egg+IVF and became the legal parent. It wouldn't surprise me if Japan just made an exception to screw over lesbian couples, but I'm not sure if anyone has actually gone to court about shared motherhood yet.
Of course, even if such an argument were successful in court, it wouldn't help gay guys like our protagonist. Ultimately Japan should just legalize gay marriage/adoption. But I do find it interesting how technologies like IVF can help illustrate the absurdity of bigoted laws.
Edit - To clarify since IVF isn't legal for gay couples in Japan, the couple in this hypothetical could go to another country like the US to have the procedure done, and then return to Japan pregnant. My understanding of Japanese law is that fertility doctors are barred from assisting gay couples, but a gay person getting IVF somewhere else isn't breaking the law. As such from a legal perspective such a procedure would be valid I think.