"I do feel though that 'sharing' my partner - whoever it is - is kind of something I'd be Ok with. Clearly I'd want to know where they were and so on but I understand what it is to have feelings for other people, close friendships, etc - I dunno, but sometimes to resist is very painful. There is a huge spectrum of closeness between people."
Ok (sorry was out for the afternoon), lets go with that last statement for a minute or two.
You wouldn't mind 'sharing' your partner. What would constitute 'sharing'? Are we talking 50/50, or 33/33/33 or presumably 25/25/25/25? How would you feel if it was 75/25 with you being the 25? Or 60/20/20 or even 80/10/10. Would you still be ok 'sharing'?
What if time-wise, it was 50/50, but you realised that emotionally it was 75/25. Would that still work for you?
Because to me, having only read about affairs, thanks to MN, never experiencing them, it is not so much the physical that hurts, as the emotional. People who thought they were the centre of someone's world, realising that they were, somehow, not so much the centre but 'half'.
Then, more infrequently, we get a woman who is brave enough to admit to being an OW. She, despite knowing that she is the OW (and implicit with that is the knowledge that there is an 'other') having started off feeling OK with the arrangement, soon feels the imbalance of realising that although he may sleep with her, and spend time with her and sneak out to talk to her, there is another woman who he will not sacrifice.
You see, I think the idea of 'sharing' starts out reasonable, but emotions are emotions. How many best-friend sagas do we hear about, when there were 2 best friends, another friend is introduced, and then the first friend finds they have more in common with the third friend, and friend B feels pushed out?