Silly or not they're still advlt themes and it doesn't make them any less mature. I would like to point out that in Chaucer's Canterbury Tales, the Miller's Tale includes a fart joke and some out there six jokes. It if's good enough for literature, it's good enough for a Fallout game.
In all seriousness though. No, the sixual themes and content were not what made Fallout 2 or Fallout 1 good. It was the writing that did that (and lets just say that I'm greatful that Obsidian's doing the writing and not Bethesda). However the inclusion of sixual themes in the Fallout games helped present the humanity of the setting. six is a constant presence in our society. It shapes our buying habits, it covers every form of media, and it shapes our perceptions of the world. After survival (ie: not dying) it is the most powerful driving force in determining human behavior and cognition (I should know, I've got a MA in psychology and I'm working on the next degree atm).
So, what do you reduce humanity to when you strip away that most fundamental of forces? Nothing.
Part of the draw of a post-apoc setting is the regrowth aspect of the genre. The world is destroyed, civilization has collapsed, but humanity still survives. People still live, love and procreate. That's why six has always been a fundamental aspect of the post-apoc setting. Often it's displayed in 2 different forms. You have those who objectify six and use it as a tool to survive. Prostitues or the sultry women who manipulate the despotic warlords from the foot of their thrones. On the other hand you often have "pure" female character who represent growth and rebirth. They are the farmgirl love interest the hero must inevitably save. (I'm not making this [censored] up, watch a couple of old Post-Apoc films and call me a liar). Not the most pro-feminist of concepts I'll admit but it's a defining characteristic of the genre.
Look at Fallout 1. Tandi fits the mold perfectly. The sweet, pure farm girl you save from the sadistic raiders before they can do anything to her. And what do you do with her? You leave her behind in Shady Sands, a town that pretty much represents the rebirth of humanity in the Fallout series (it's all new. There are no recycled parts or ruins in it's construction. It bears no resemblance to the tech of old America.)