I know what your point here is, but my short, quick list was made with the starting point in mind that Ian and wild Holly are now becoming a couple. It is clear by the way it happens that the main drive for Holly to become a wild, slutty girl was to get more appealing to Ian. Now that she has reached that goal, I doubt her drive towards her slutty side will stay.It kind of depends on what you consider a douche or loser, though. For example, your possibility of c) presumes both Holly and Ian will be happy to "develop Holly's wild side", but it can also mean Holly will want to have sex with other guys, and the player may decide that no, they don't want their Ian to agree to that and that he wants a monogamous relationship the slutty Holly would chafe at.
Hence my theory A that Holly stops further experimentation on the wild side and reserves her sluttiness for Ian and their bedroom. Having reached a sweet spot both like as it is.
Theory B is that both Ian and wild Holly experiment a bit more, nothing big as said, but still some further development, both being comfortable with the "new" slut level reached. They are a loving dedicated couple.
And Theory C is an Ian who really likes his girlfriends very slutty and open to more things.
You Theory D is not a wrong option per se, but more of a bad end follow up of C where they go too far. And as I wrote in an earlier post, I think we will get some event decisions how far Ian/slutty Holly will go or keep it at this level.