The Sims 4 is all about daily life and everything that comes with it. You create your perfect people, have them go to work, fall in love, and achieve their goals. It's about as close to real life in a video game as you can get, and comes with all the processes that are involved, such as getting butt naked and doing a Woo-Hoo to make a baby.
Obviously, The Sims 4 does its best to cover things up and make things as PG as it physically can. You can't see much in The Sims 4 when you're getting down and jiggy with it, just some rustling under the covers and maybe a limb or two poking out. That's where mods come in though, as a mod called Wicked Whims has been around for over a decade to spice things up in every Sims' bedroom.
Not only does the mod add full nudity to the game, but it also adds a lot of raunchy positions for Sims to get into, along with menstrual cycles, fertility tracking, birth control, and STIs. Not all of it is filthy, but I can almost guarantee the majority of people that download the mod...
