Hello VioletElle,
BH here. As others have stated it is important to be totally honest with your BH about details, but I'm not sure if they expounded on why.
What you did with OM may have been sporadic, routine, plain vanilla sex, but in your BH's mind it could be, and probably was, the most wild raunchy, uninhibited, swinging from the chandeliers sex imaginable. Even if it was not, which it probably was not, his own thinking will fill in the blanks BADLY.
I know it may be uncomfortable, but endure. Push through no matter what. Consider it an act of mercy.
I was harsh on my XWW, to my regret and shame. Yes I had been betrayed and humiliated but it was no excuse for cruelty, even though she was cruel to me. But I HAD to know.
At first she was reluctant, but OM, when I confronted him, forced her hand. He bragged about things she did and said, much of which was true, but he embellished quite a bit too. There was no way for me to know in the early stages what was true or not, so with me ready to blow up the world...her world...she came clean. It hurt like hell, but it was also cathartic.
Questions asked and answered:
Did you ever deny me sex because you'd finished with him and were tired/satisfied or because you were planning on being with him and wanted to be 'faithful' to him?
Did you ever have sex with me AFTER you'd been with him?
Did you ever kiss me or let me kiss you after you'd given him oral?
Did you ever have sex in our home, car or special places?
Did you ever climb into bed with me with his residue still on you?
Hard questions. Harder answers.
I'd suggest anticipating his questions about details and writing the answers down. You won't get them all so don't worry, but try to have some explicit answers ready. Make some sort of timeline. Be prepared for him to ask questions over and over again. This is a marathon not a sprint. Healing can literally take years and often the 2nd year is worse than the first.
My XWW and I divorced but we're still a couple. Our relationship is different, but more honest and open. For a while it was like she was trying to outdo what she did with OM. While good on the surface, it wasn't really honest. She was trying to prove something and I felt it. We've grown together and now things are 'real' between us. Be prepared for that also.
Your BH might want things 'wilder' in the bed. Give it to him, if you can. Sex is healing.
I wish you luck. Keep posting.