At first I also dismissed it as cuckolding. I thought NTR fans liked to insert themselves as the cuck, and wondered who in the world would like to experience the dispair the protagonist goes through, that's why I always regarded it as one of the darker genres of hentai. But it turns out that it's not as bad as I though, and I ended up liking it a bit. You don't necessarily need to self-insert as the protagonist, maybe seeing his dispair is part of the fetish, and I admit that seeing the stolen girlfriend enjoying it is kind of hot. But I still don't fully get it.
>It's basically a cheating power fantasy. I think it only gets a bad rap in the West because it's disregarded as cuckolding, which is far from the truth considering the girl's partner doesn't even find out usually and often isn't even shown. It's just cheating + moral degradation and a potentially some mind break.
Well, I think in those cases it's more Netori than Netorare (although one could still self-insert as the villain in the latter). Netori doesn't seem to be very popular, though.
I've heard the Germans are really into bondage. A kinky people indeed.