/jp/ - Otaku Culture

It would largely depend on the intent of the youkai in question. At least some of the yokai have shown an ability to blend with humans, with the vast majority of local humans none the wiser. You could have a marriage with a girl and live your whole life without ever knowing she was a youkai. Yeah it might be weird she doesn’t age, but hey, she’s Asian. Or she could adjust her appearance/illusion to fake her aging.
Assuming you know about what she is and perhaps even pursue her with that knowledge, a lot more possibilities open up. As >>40104811 said, maybe she wants to extend your lifespan to equal that of your wife’s and either directly gift you inhumanity directly from her or use other resources to bring about your ascension. Fortune Teller became one through his personal magic book and his suicide. With a literal who achieving it with relatively low material cost, it would be laughably easy with the backing of an immortal wife and the knowledge and instruction she could offer about how to do it. Some youkai (vampires) are contagious, and for everything else there's likely either a tale, book, or a scroll with the needed details.
Assuming you choose to stay human it could prove to be a one-sided relationship. She probably assumes she knows better than you on most things as she’s been living her life the same way for so long. She might be very insistent on how you do certain things, and you’re not going to be able to argue the point to someone so much more powerful than you. On the other hand she might also be accepting that as a human, you’re going to do stupid shit, and you’re going to learn, only really interfering when you run a serious risk of major injury or death.
Having children could be complicated depending on what degree of nurturing instincts she has. Also the question of how you are going to keep your likely very powerful half breed children in line. You may have to take up some youkai slaying arts to spank your kids when they start misbehaving or testing your authority. Albeit maybe having kids isn’t the nicest thing you could do to her, as they will also age and die, if somewhat slower than yourself. By year 500 her husband would have been dead for a while and her kids would be constant reminders for her, as they start to reach the limits of their lifespans as well.
If she likes you but is disinterested in her children it could go two ways. First is they just learn how to interact with humans from daddy and they figure out the youkai half on their own. If your kids develop half blood traits later in life or they manifest during stressful situations you could be left handling an incident with second hand knowledge. Your daughter comes back with slit pupils and a set of serrated fangs, crying because she bit a chunk out of her boyfriend in a heated moment, and she doesn’t know why her body suddenly grew extra organs and a taste for human flesh. Or another youkai decides while her mother isn’t in the area that the half breed girl’s liver would probably be more filling than a regular human’s. Your daughter fights it off but the powers summoned to win the fight leaves her with a drastically changed body and craving for violence, and all your wife bothers to say is “oh that’ll fade once you turn 50, the bloodlust I mean, the physical changes are permanent,” before returning to her book, and now you have to explain it more in detail to your kid with only second hand knowledge.
The more I type this out the more I think just becoming a comparable youkai is a more stable way to manage such a relationship.