On the subject of whether I would share a regular partner, the only answer has to be it depends...on her, the other person or people and the circumstances...I have shared an ex in mmf and mfm...she liked boys and gals...and while not bi, I was cool with the guy being around, his cock was a bit smaller than mine, but good sized, so guess that helped me not
to feel outdone on that front at least...and she mainly wanted a cock to suck while I fucked her...we had agreed we would do both...and can only say that watching her with another woman was worth having the mmf..and that was fine really...as she told me after I was still the best she had ever had...we did each a handful of times if that...it was all fine and fun, and no hard feelings about the hard feelings.
The other sides of the four sided coin ;-) are cuck or straight or bi couple mmf or the hubby can't anymore..can't think of more...have experienced three but not bi as not my thing...
1. Hubby can't anymore - A couple of simliar couple experiences...in main case I know he struggles with it but he cannot get an erection anymore so he gave his wife permission to take a lover...on grounds she promised not to leave him. She is such a lovely lady she would not anyway..he was never really there (in fact overseas for much of the time), so not cucky at all, though she did say I am considerably bigger than he was when he could rise to it. She did say part way through that it was meant to be a fling but she had fallen for me more than she had thought she would...but we are grown ups...we had lovely times...she had to move abroad with him for the climate, and we may see again, or not..but it was as cool and pleasant as probably could be. I met him a few times and he was very gracious. She did not tell him the details and he did not ask..so she did not rub in that she was fonder of me than he may have liked.
2. Cuck..done couple of times with couple of couples. From what I have heard it is most often the hubbies or male partner who initiates...but often the woman takes to like a duck to water...I guess as mentioned before some women really do derive extra/more satisfaction from a big cock - lady in story 1 said it makes her feel like she is with a stronger more virile guy and that turns her on.
In one case however it became clear that either hubby had wanted and then realised not, or she had initiated and pushed him. As he sat watching and sort of playing with himself in the corner, she got more aroused and as I started to enter her moans were loud...he got up with huffy noises and stomped out, slamming the door. Well this kind of put me of my stride...I said I was not comfortable, but at this stage she wanted me to finish her off so to speak... I tried and she did orgasm, but I did not, and suffice to say did not return.
In case study two
Completely different. Super relaxed couple...no edge...apparently his main thing was it turned him on to see other guys attracted to and excited by her...it was maybe not heavy cuck...as in no real humiliation...but he did say a few times to her...look at that big cock going inside you, and she would respond enthusiastically..and she said a few times how good it was feeling and told him to look at one point...he just laid back and played with himself happily most of the time..he assured me it was nothing bi on his part...just pleasure for his good lady and that they are strong together ad still have lots of good 12 1 sex so he has no problem sharing. I do not suggest this is the most common scenario!
3. Straight mmf...not much to say really except fine if both guys not wanting each other...some equality of bod/cock to avoid jealousies...fair shares...safe! ...and no stupid games behind backs...def would not want my missus off fucking someone else alone and coming back and showing me pics or describing what went on...but as long as comfortable she is 'with' me, and if I am not comfortable she will back me up,,,what;s to stop!