to be clear.
after i got pissy about her sleeping with other people for the 800th time, she came to me in mid-2004 and just laid it down on the table: she wanted to drop the bullshit and officially declare that we were in an open relationship. she was tired of these arguments.
but, she didn't want a break-up. she was very, very clear on that point. what she wanted was for me to accept that she was going to do what she was going to do, and kind of learn to live with it sort of thing. which, i suspect, is what she'd been angling for the whole time. i don't, however, expect that she would have accepted any kind of infidelity on my behalf (something that i was never even interested in - i'm a romantic, i'm monogamous).
from my perspective, i had been hoping, to that point, that she'd grow out of it, basically. so, to have her sit me down and say "this is who i am. i'm not going to change." was a wake-up call, and i did what i always did: i pulled out.
there are underlying factors. i felt insecure about the potential longevity of an open relationship, although in hindsight i could have answered my own question - we'd been in one at that point for two years. why couldn't it last other than me not wanting it to? i was nonetheless fearful that she'd end up pregnant or something, that an open relationship was not sustainable in the long run. and, that itself masked the most dominant underlying issue: at that point it was clear that she really, really wanted kids and i really, really didn't.
but, i didn't hate her for it. i accepted it. and, we stayed friends - in fact, with benefits - for many years after the fact.
we called it a mutual break-up, but i think it was almost the opposite: neither one of us really wanted to break-up, but we both realized there wasn't really a way forwards.
we were living in a one room apartment at the time. she wanted to maintain the arrangement, which in hindsight suggests she wasn't really serious about breaking up. i mean, how do you process somebody that tells you they want to break-up, but wants to keep sleeping in your bed? she expected me to accept the open relationship, and was baffled when i didn't. and, i ended up moving out unilaterally.