Dude i can't be the only person who's bothered by this. She doesn't get "stolen" from you, she leaves you. She's a person, not a bike that you leave in a bad neighbourhood for too long, she decides to leave you, even if it's a stupid ntr porn game. That bike cuts it's own chain and padals itself into an ugly bastard's hands.
But i agree, ntr should always be avoidable, otherwise the woman is just a monster. Imagine if choice-based RPGs didn't give you a choice and you must play as an evil asshole, i can't imagine the fun in that. With ntr games i always do an ntr run then i do a wholesome canon run. I only do the ntr part because it's a stupid fetish of mine, with real games i only do good-guy playthroughs, i don't like to be mean.