First and foremost, try it and make sure you get "the mod." There are versions of it out there without the questionable content or you can disable it completely.
If you aren't convinced by an imperative and a 5 star rating, well...
Free Cities is a game, first and foremost. Unlike the vast majority of stuff you can find on this site, Free Cities is an actual game with challenge and strategy. Ultimately, yes, it is a sim and once you've figured out most of the difficulties you'll face your time will probably be spent trying to create your own little future degenerate haven, but from start to finish Free Cities places most of its focus on the gameplay.
So what gameplay is there to offer?
Your primary goal is to make money. You need money to run your city, to feed and house your slaves, and to invest in protection and the future prosperity of your city. There are a few primary ways of doing this at the start and the options open up tremendously as you go on, but by and large slaves make money. Better slaves make more money. If you are whoring slaves obviously prettier, younger, and more sexually experienced slaves will bring in more cash than sick, old, and ugly ones. You can "upgrade" slaves through long-term hormonal treatment or quick-and dirty plastic surgery.
But you need more money! So it's time to open up a proper Brothel, but who is going to be supervising it? A smart, loyal slave would be best, and if you really want good results, cultivate a dominant personality for your madam.
You will typically be breaking in and training a few slaves at all times while increasing the quality of your loyal slaves, all the while saving up for expensive upgrades to your city and properties. Eventually you'll start running out of options to pump up the productivity of your slaves and you'll need to start changing the demand side of your economic balance.
Managing your culture is the second main focus of the game and has a huge effect on what kind of sexual content you see in between turns. The idealogies you select could be as simple as a "plastic fetish" which would turn the typical distaste for surgically enhanced slaves into a massive buff for their attractiveness. You could get a little more degenerate and make a particular race "marked" for slavery, earning you extra respect and fame for owning slaves of that race. Then there are the incredibly game-changing cultures like Paternalism, which focuses on education and gentle treatment of slaves, prohibiting harsh treatment but in turn making slaves very loyal very quickly.
Once you've got the hang of the game, deciding which culture you want to focus on becomes the main draw of the game. I've made a city that worshipped perfect, unaltered, amazonian physiques. I would hold a massive gladiatorial tournament where the eventual sole survivor would be treated to a lifetime of pampered care and added to a harem of superhumans. I've made exclusively trap arcologies with all of my slave leaders big-dicked futanaris. I've made Paternal cities and adopted blind, mind-broken paraplegics and restored them to perfect health. Bimbo nuns, Egyptian cow-girl farms, industrial sex factories, it's all here.
Free cities has incredible scale and one moment you can be struggling to balance the hormonal levels of your entire brothel and another you'll be delicately (or not) handling a slave having a mental breakdown in the shower. Of course, the game naturally encourages you to buy as many slaves as you can, which leads to a necessary reliance on the built-in automatic slave manager. I've had great success using it, but it does require some practice and study to use. Unfortunately, most of the strategies that rely on fewer, higher quality slaves take a good deal of experience and as such I'd recommend exploring the slave manager with some simple rules for keeping them from getting sick or fat.
The endgame is definitely a "it is what you make of it" deal. There are some interesting events that play out and some options for making tons of money to open up suboptimal strategies for slave cultivation. Many of the late-game events only have marginal connection to the slave-management side of the game, but they also do a great job of worldbuilding, of all things. The world of Free Cities is a post-apocalyptic anarcho-libertarian degenerate hellhole with the exception of your surprisingly despotic city. Or, if you decide you like the idea of a sex arcade, it's even worse than the rest of the world. To quote a certain IGN meme, Free Cities really makes you feel like a Mad Max dictator with an unusual predisposition towards body modification. It's a great experience and stands head and shoulders above sim games even outside the adult genre. There's simply nothing quite like it and I don't think we'll see any real competitor for some time.
10/10, would enslave my aunt again.
