The best NTR for me are when the MC is a chad, he does everything right, hes got a massive dick and the girls love him. But he still loses to the ugly bastard somehow. (Without blackmail/mind control)
Its no fun unless the wife gets seduced
I strongly agree with both of you on the whole "No blackmail/mind control" and preferring seduction thing. NTR is a very personal fetish to me and blackmail, drugs, hypnosis, magic curses, mind control, mindbreak, and other such things just remove the personal feeling from it. Since the girl getting stolen isn't even in control of her mind/actions anymore or isn't even the same person anymore.
Its a lot nicer when she gets stolen/seduced via personal flaws and/or mistakes. Flaws/mistakes of either herself or her partner. Or just unfortunate circumstances.
Like high-libido girlfriend can't go without sex from her boyfriend at least 3 times a week. We spend a while getting to know them, their relationship, and come to like them. Unfortunate circumstances arise that separate them for months or even a full year and she's now around playboy all the time during their separation. Playboy can't get any fuck in this situation either. Girlfriend and playboy end up talking and finding they have similar hobbies/interests, spend casual time together, sexual frustrations build, and one thing leads to another until they start fucking.
Management games like Welcome To The Sexy Bar are also nice since the NTR only happens if you, the protagonist, are bad at managing the bar. Which makes the NTR reactive to your flaws as a manager instead of tied to the whims of some kind of magic plot item.
Either of those are a lot more interesting to me than like 2 pages of "I have hypnosis phone/sex magic/hentai aphrodisiac/photo of you doing so-and-so. Fuck me now." and she just goes "Ok." And then the sex is identical to the sex in a non-NTR story with the same lead-in/justification for the sex.