"Stealing girlfriends" is a metaphor you fucking idiot. It pretty much means running off with someone elses partner.
Hint: Spot the feminist.
There is a big difference between kidnapping and willfully going with someone. Unless the original poster of the, I got my girlfriend from a love triangle, post is a sicko, I think it means that his wife CHOSE to be with him, which means her old boyfriend was probably shit anyway.
And "Stealing girlfriends" isn't a metaphor, it means what it says, however the term steal doesn't really fit if its the stolen's choice, now does it. I've never heard of a Flat Screen TV CHOOSING to go with the thief, I've never heard of a Car that becomes the thief's getaway driver because the original owner is a brutal appliance murderer.
Cheating is wrong, period. However, in the case of a Threesome, if thats what the op actually ment, then its a choice between all three people, and not cheating unless it continues without one of said three. A love triangle, as what the op actually said, doesn't have to involve sex in any way shape or form for a relationship to blossem for the "Woman Objectifying Male Thief" to become the choice for the woman in question.
It all comes down to, none of us know what the details are that the op went through, so STOP ARGUING ABOUT IT AND MOVE ON WITH YOUR LIVES!