What do you mean by "openly?" I mean, if the parents are bedecked in leather and spanking each other while using nipple clamps at dinnertime on the table, then I'd say I'd be opposed to that. Then again I think parents should be discrete about their sex life in general, at least do it behind a locked door. Walking in on one's parents doing the act, in whatever shape or form, can be pretty traumatic to a child. If not emotionally scarring, then definitely under the "I don't want to see that sh-t!" category.