Honestly, just from the ones we know about I assume the casting couch is kind of the norm, not the exception, in Hollywood. Mutual Assured Destruction and/or NDAs keep them all silent. Look how even when someone is caught redhanded VERY few celebs make any sort of comment on it.
Given that for pretty much every actor, there are literally a hundred who are just as talented, just as good-looking, etc., it stands to reason that if it will get them the part, SOMEONE will do it. The fraction of people willing to make the trade is likely higher than the ratio of roles to adequate candidates.
I mean, if it would get me a 7-figure income, I don't think there's much sexually that I wouldn't be willing to do, especially if only a few people who keep their mouths shut are going to know.