For you guys who doesn't know, swinging/swapping genre was a long forgotten theme before NTR even became popular. ( At least I haven't found any new one)
Usually its start with a couple that bored of their sex life and wanting more spice.
From there it would usually go 2 divergent path, either they become more passionate again after becoming jealous seeing their partner fcked by other people or it can go full NTR route after their partner discovering the other guy/girl is much better than their current partner (but this instance was very rare at that time).
NTR have 3 different instances,
Netori - MC stealing other people partner (Usually the great old H anime mainly using this theme)
Netorare - MC got their partner stolen (Not just fcked, their partner fall in love with the one stealing from MC, infested the market rn, some are good but most of them are garbage)
Netorase - MC consents to let their partner be fcked by other people (We are seeing more of this as it possibly invoke the most emotional reaction from MC and reader, most stupid imo but can be believable if done correctly)