The word abuse is definitely thrown around too easily. I don't think this is abuse at all, maybe very wrong in this day and age, but I'm very much sure in times gone by it would have been completely normal. After all, sex is a basic human need for our species to survive.
I've heard (not seen, but I'm sure many kids have caught their parents) my parents, and it didn't do me any harm, just grossed me out lol