Do you know of any manga/game/CG where the MC gets a chance with the girl given by the NTRer and fails miserably? I mean:
1. The NTRer steals the girl
2. The MC finds out
3. The NTRer gives a chance to the MC to prove he is better than him in sex.
4. The MC tries his hardest and fails miserably
5. The NTRer keeps fucking the girl (probably in front of the MC)
6 The MC falls in despair
Something along those lines :D
[oyako rankan]
[スワッピングパーティー]
[壁の向こうの妻の嬌声1&2]
[彼女の肢体に付いたアイツの唇痕]
[新・他の男の精液で孕んでもいいですか]
[beat blade Haruka]
[beat angel escalayer]....
hmm i have a few some more of that but they are in a pool of NTR games both disks in my PC, but i think 1 night 1 game is enough for you right? :) don't push yourself too hard
[他の女の子とHをしているオレを見て興奮する彼女] i can't say much but you are main char, your main girl know you are having relationship with another girl, and she like to watch it..... you bang both :)
Anime : [Accelerando], old one, but in the end main char's dick win :)
[I can] this hentai anime have game, LiLiM made, but NTR = ending, not like anime, your main char still convince your girl with his "peanus" but failed because he has a "soft one"
i watched these long time ago ( 5 years ) so i don't remember much but i can do a quick summary
[Hitou Meguri Kakure Yu] story about mc back to his home town, meet his girl , she is running a hot spring, very popular,he has in his mind gonna marry her, one night he heard a moan voice in the hot spring, he saw she having sex with camera man ( this guy do advertise for the hot spring that why it popular ).... so check if i'm wrong or not, ty
[Hitou Meguri Kakure Yu Mao Hen] a couple on a trip, main girl got raped by other man.... i don't remember this is NTR or not either..... lol, check it for me
CG, i have some, but only if you can dig something in this mess... you can try
also you are asking for sole relationship genre, try to find some :) hope i can help something my friend
( i only give you name of some games, search it yourself, i don't do charity )