I think the shock factor is not actually that Regina would do something like this, but that David is involved. Because as far as the whole “dark territory, not-so-kid friendly, sleeping around thing” is concerned, we’ve already seen Regina’s rape/manipulation of the Huntsman/Graham. We’ve glimpsed the depths to which she is willing to descend to get what she wants–we just didn’t expect that it would lead here.
Random moment: A bizarre image crossed my mind of Regina giving David an enchanted apple that messes with his head in an almost Garden of Eden-ish sort of way… 😯