Well, I have zero experience in either, but I’ll have a crack at answering.
Strangely enough, I’d say that A is less acceptable, but B would hurt more. I think the reason is that in the case of A, there would be less of “Alas, I’ve lost this wonderous person” and more of “well, I guess they weren’t so wonderous as I first thought”. I suppose rationally there shouldn’t be much difference in my reaction, but I’d probably be focussing more on the fact that I kind of dodged a bullet. In the case of B, however, I may well still actually like the person (depending on the amount of dishonesty involved, but I don’t think there’d necessarily be much), so I’d feel the loss more.
I don’t really think there’s a difference in pain between the two. Either way, our hero is ground into the mud, and must find another path to happiness. In one catastrophe, his love is found to be inferior to some other guy’s. In another, his lovemaking is found to be inferior to the interloper’s. Either way, he must claw his way up from the depths of defeat and inadequacy. Either way, it was not to be. If you hook up with a person you weren’t meant to be with, a breakup is a triumph. A breakup later on would have been even more painful.
In my experience B usually turns into A sooner or later. A friendship is one thing, a deep meaningful relationship with someone is something else entirely. A makes it pretty easy to get upset and, depending on the situation and the relationship, get rid of her. B is a more complicated and potentially messy situation. My worst breakup ever was a B to A situation. She said he was just a friend, he ended up being more. At least she eventually married him, so I wasn’t being dumped just for a crush or a hump, but the emotional jerking around I got before the breakup was the worst hell I ever went through.
If I was subsequently dumped in either situation I’d be pretty upset, but A would probably be the worse of the two. That kind of implies that sex was the reason for the breakup, and I have a bit of ego tied up in my performance as a lover, whereas situation B might be something as simple as meeting someone who matches her personality better. My personality isn’t something I can work on or improve and so I could be more fatalistic about it.
Being polyamorous, neither A nor B would bother me, provided that everything was out in the open and it wasn’t a prelude to my partner breaking up with me (hey, it happens in “poly” relationships, too).
I think B would hurt more than A, but I’d find it a better reason to come to terms with if that was why I was dumped. Something purely physical destroying my relationship? Crappy. Someone realizing that they had a true, emotional bond with someone other than me? Would cut me to the core, but I’d be a lot more inclined to accept that it was the way things had to be.
Along the same lines, if my SO were to have sex with someone, I’d rather it be an emotional thing than a one night stand. I could much easier understand throwing away a relationship because of love, rather than just some physical fun.
Hope that made sense. Summary: B would hurt a lot more than A, but it would still be a lot better.
“In which case would you be more hurt?”
B) They develop a deep, emotional bond with someone else, and although the relationship is platonic in the strictest sense, they feel more than just friendly affection for this person
That’s like asking which would hurt me more, for her to eat cabbage that someone else cooked or to listen to music that I didn’t write.
You monogamous-exclusivity folks are so fucking weird.
<Insert usual disclaimers about different strokes for different folks and being nonjudgmental>
<Insert usual acknowledgment that if I do not wish to be considered and dismissed as weird for not being monogamous-exclusive I shouldn’t dismiss those who are as weird>
Umm, to me, I mean, you’re so fucking weird to me.