TLDR; I find the lack of brothels unimmersive.
Warning, philosophy, anthropology and sociology ahead!
I consider it this way: Fallout is all about surviving and thriving in the place of a total sociological breakdown. Society has been completely blown away and now it's our job in Fallout 4 to begin the recovery. Fallout 4 is a fantasy about the worst parts of the human experience happening all at once and exploitation of sexuality is a significant but in the end minor part of this struggle for whatever is left. In this setting, sex and the control over others it brings becomes a barterable currency. The lack of it in game is silly to the point of we see all this death and destruction and the decay of what has come before but this one absolutely fundamental part of who we are is almost entirely ignored. Human nature is human nature, there will always be those that will exploit others for personal gain. The lack of this sort of depravity is as much of an immersion breaker as the radioactive water. It's a vile nasty cesspit of a game world with rampant cannibalism (you even get a perk for that) and the survivors scrapping over the dregs of what was once a functioning and mostly harmonious society. Against that background, prostitution is inevitable and a natural outgrowth of the starter conditions.
In the real world rape and sexual exploitation are horrible life destroying things, as is rampant killing, radiation, complete disregard for life of any kind, etc. Leaving it out of game is understandable from our point of view inside our cultural reference but far less so when taken in context against the background of the in-game world. This is literally why it breaks immersion. The reality is that the survivors of the war and their descendents, given a world such as what the Fallout series depicts, should by rights be an illustration of the many facets of PTSD on a large scale.
This game is surprisingly morally clean given the world in which it is set even if prostitution is implied but not really depicted. When the laws of society break down and currency no longer has much value, you barter what you have and if all you have is yourself, you do what it takes to survive. It's not a place a sane person would want to live and the surprise is that the horror has been allowed to exist this long, not that it ever existed in the first place.
Most of the raiders and larger gangs appear to be more or less sane. Real humans don't live this way without attempting to improve their lives but for the sake of the fantasy world, we play along. With increased levels of resource availability, settlements should see an immediate uptake in recruitment everytime a raider band comes by. The smallest groups wouldn't be attacking, they'd be surrendering as individuals and hoping the good guys will share some of the largess.
Look up what the Mongol hordes did if you want to see what should really be happening with the Commonwealth in the face of the hordes of raiders. Where are the raiders coming from anyway? The sole-survivor has probably mowed down more raiders in game than there were people in Boston prior to the war, is there some kind of raider cloning facility out there somewhere? Why are they raiders anyway? What does being a raider provide the individual as resources become increasingly scarce? Why haven't the raiders either wiped each other out or consolidated into larger groups for mutual survival and protection, thus becoming basically the same thing as the Minutemen? Why didn't the settlers band together and stop the worst of the groups long before the survivor even came out of the freezer? Fat Man nukes are a thing... Get a couple of vertibirds and carpet bomb Nuka World so it glows in the dark and I guarantee the raiders will knock it off after the first overwhelming show of force. Repeat as necessary.
I won't get into what really happens with groups of people when large scale disaster occurs in the real world, we'd be looking at an entirely different and way more boring game.
Sim Settlements is really the implementation of the start of the consolidation of disparate small groups and the rebuilding of functioning society that should have been happening organically beginning at the point of the bomb. This begs the question lore-wise vs realism of why didn't this healing process start happening as the smoke started cleared instead of 200 years later? Better late than never I suppose. There's ghouls around from before the war, they would remember and be able to point the way back from the abyss if nothing else. Hancock would be an excellent catalyst to rally around.
In broad strokes this is why Sim Settlements is so lore enhancing and why it changes the game fundamentally for the better. We go from a sense of futility at having created small groups of settlements too stupid to maintain themselves and need to constantly be micro-managed all the way to the hope of being able to catalyze and direct the rebuilding of the world from the ground up. A noble goal in an ignoble world.
Just my 2 cents and some of the philosophy I RP when I'm playing the game.
