I think princess_stomper has a very good point, there. Looking back, Fallout 2 was probably over the line in terms of gratuitous sixuality and "mature" concepts. But on the other hand, a fair portion of the more extreme examples did take place in Reno (which is where you tend to end up if Vegas considers you to be too "sleazy"...
) So to an extent it did make more sense within the context of that location, than it would in DC.
If I'm being honest with myself, Fallout 2 was closer to a young boy's idea of what "mature" would be; rather than it's traditional meaning. Just tossing these elements into the game isn't necessarily going to make it any more gritty or desperate; I think it's more a matter of quality over quantity. (And that said; I'm sure the vast majority of the "pro" side of this debate feel that same way - obviously no-one's really asking for gratuitous boobs for it's own sake, etc.)
When I first played Fallout 3, I remember "hiring" Nova (that's the hoker's name, right?) when I first got to Megaton; and being a little bit... dissapointed with the outcome. I wasn't expecting anything more than the classic fade to black, but somehow the whole experience just felt... empty. With the added perspective of passing time, however - that's actually sort of spot on for something like that, though. We're talking about something that's really more of a business transaction than anything. The screen goes black and opens again with you and Nova standing next to the bed. Nothing terribly important happened - that's her job, after all; and she probably just wants you to leave so the room can be free for the next client.
Looking back on it now, that actually sort of underlined the depravity of what I'd just done. When it was all said and done, I was a couple hundred caps poorer, felt a little dirty, and that was about it. What was she supposed to be doing, laying on the bed with me? Cuddle time costs extra - this wasn't a romantic encounter. Was she supposed to say something clever? I'd already paid my money and got what I wanted, there's no more reason for her to lay the charm on me.
That being said, though - I do think there were some missed opportunities to show the slimy underbelly of humanity living in a desperate situation. There's hokers, sure - and that'd probably be a position many women (and men) would find themselves stuck in, in a world like that. And there were a few junkies in Fallout 3, but nothing that really showed me why that was such a bad thing. There's drugs around every corner in Fallout 3, but the only junkies you find in the game are "functional addicts" at worst. They never came across (to me) as people who have watched their lives slip away as slaves to their addiction. (Maybe this is because I've known people like that in real life; and it's hard to drive something like that home in a videdogame, but still.)
What's-his-name in Megaton seemed like someone who maybe had the potential to one day hit rock bottom, but the only real indication that he had any sort of a problem was because everyone was saying he had one. There were a couple of people in Rivet City that seemed to have a drug problem, but not to the extent (that I could see,) that they were trading in their pride and humanity for just one more fix.
I think, if anything, that's what I'd like to see more of in the next Fallout games. It's not about the quantity of these elements, but the raw humanity and desperation shown in those cases where it does show up. Where are the prosttutes who want to leave that life, but literally have no other options if they want to continue to survive? Where are the junkies who have completely given up and are willing to do anything at all for the promise of one more fix? If we really want to go the mature route, then the focus isn't about having these things in the game; but the human cost these things carry with them.