Hmmm. Maybe they get depressed because their boyfriends are poor lovers ;-)
Deep down they know they're just a warm place to put it to their pimply adolescent boyfriends. Feeling used must be awful.
There is probably more truth to this than you realize. If it is even possible, it would take a pretty good lover to make sexual intercourse feel good the first few times for an inexperienced young woman. Think about it, the inexperienced young woman thinks that intercourse will be this super wonderful experience but instead it *hurts* when compared to the foreplay (hopefully) that lead up to it.
That alone would tend be somewhat depressing.