When you decided to open up your marriage was this kink exploration a big factor for you? Was your husband aware of this going in and ok with it?
Does he have other partners and has he come home with scratches or hickeys? If so was it a big deal?
I don't think second guessing why he's upset is a good idea. I'd ask directly if seeing the bruises triggers bad memories and old issues.
Assuming the bruising can be covered by clothing why not negotiate a break from activities upon return from these kinds of dates a day or 2. That way you wont feel rejected and he wont have to see fresh evidence of a beating and get upset.
Annabel had a good point ....going off to be dominated.... getting a need fulfilled you can't get at home .... coming back with marks and bruises ...it maybe getting into that cuckold area. It's worth asking or talking about.