Bovine manure, it’s exactly what happened here. The fact that the relationship was ‘de facto’ monogamous is beside the point. The OP made it clear from the start that he would only promise to tell the basic details when/if had other sex partners. He didn’t promise -not- to have other sex partners. There is a huge difference in the level of emotional investment each side is making. The OP’s partner made it clear that, while not 100% happy with the promise, she would accept it, even though she would probably be hurt bad enough to want to end the relationship.
As such, the only reason the OP didn’t have other sex partners during the time was not because he was fully committed to Herself - which is what monogamy is - but because ‘he didn’t want to hurt her’. In other words - the very excuse he so derisively dismissed those of us who are monogamous! As I said before - just because the OP is incapable of making such a commitment, don’t insult the rest of us who can and do.
He called this ‘his experiment in orthodox monogamy’, but that is a pathetic joke, and it’s also an incredibly insult to Herself - I’m sure she’d be thrililed to know that he’s describing a 12-year relationship as ‘an experiement’. But monogamy is not just about not putting your dick into other women. It’s about committing fully to one relationship, and only one relationship, because that’s what you want to do, not out of some sense of guilt or sacrifice.
I don’t call myself a vegetarian if I pass up a steak at dinner and have the salad bar instead. Just because he went a period of time only in one relationship doesn’t mean he has a clue what monogamy is, and this relationship was most certainly -not- monogamous, since both parties did not start the relationship on the same terms.
Look, I’m sure the OP is a great guy to go out to have beers with. But I wouldn’t want him dating my sister or any of my female friends.