That’s called Netori (taking another guy’s woman). You’ll NEVER see this being done from a protagonist’s perspective UNLESS it’s a loveless relationship and the protag hasn’t done anything really wrong by taking someone who didn’t actually belong to anyone else in the first place. Moral law gets in the way. I can only think of ONE example of actual Netori by a MC, ie, a loving wife in a happy marriage taken by the MC, and that is in a VN called “God’s Game Six People Confined” or something along those lines. It’s a porn death game run by a supernatural entity calling itself “god”. Even though it’s porn it has some of the very best character writing and emotions in anything made by Japan I’ve ever seen. Basically, the characters’ old personal relationship break down while in the pressure cooker of the hellish death game that lasts like 2 weeks but every day is like a decade of life-and-death struggle. it’s not just goofy fake porn that happens just because. There’s also a sequel WN called “Devil’s Game” (not porn, a traditional death game) but last I checked it didn’t get a VN adaption.