"We have an open marriage, and I was not being an unethical assmidget" is a reasonable conversation to have.

And it would be best if the mom validated -- she doesn't have to disclose details! just acknowledge the open marriage! -- because otherwise it might sound like Just An Excuse. If I caught a guy cheating and he said "oh it's okay we have an open marriage" I'd side-eye hard; if his partner said that, it'd be more comforting.

I really hope the only reason mom didn't say anything was because of the "kid knows parents have sex" awkwardness vortex, and not more worrisome reasons...