The problem with child killing is twofold: immershun-breaking (immortal characters [that forget about your trying to kill them] FTW!) on the one hand and destroys the myriad of possibilities that could open/close for killing children in general or specific kids in particular on the other hand. I would rather there be no kids at all.
I don't know for "porn" but hookers, pimps, "bathhouses", whorehouses, and selling-yourself-for-whatever should be running rampant in such a world. Not in EVERY town of course, you must check what kind of establishment (if any) would appear in such and such part of the gameworld. Prostitution has been running around since the start of times, was there in earnest in the wild west (what NV seems to be aiming for), is certainly all around today, and would be certainly rampant in a post-apoc world, specially once settlements reach small town-size and beyond.
As for sex as a whole it can affect your experience, from the basic "get a discount" all the way to screwing the wife of local Joe and igniting a bloodshed/trial/whatever, skimming through unwanted pregnancies (do you kill her and 'solve' the 'problem'? abortion? go on with it? ha, maybe too much for a simple game, but it may be a useful sexual education campaign) and through being too dull and ruining quest possibilities with that bar owner that's been trying to seduce you.
I think FO3 had barely a few hookers here, that's stupid, and then the unkillable characters and all the gameworld changes that are not there as a consequence... I mean it is SO STUPID, it's a "M for mature" game, so what the fuck are we talking about? What's wrong with those peoples? Hello! It's computer character! Are you ther McFly?? It the same as killing that old lady witha baseball bat!! Just a computer character!! Is it less wicked to machine-gun that woman in the throat?? Computer-created and animated graphic assets! blowing an entire town with a nuke! (And all the kids that live therin!) That's ok, then? Hello, are you there??!!!