Hmmm...
So there's a 'class' of Assets for 3D modeling programs called 'personal use only'. Most game developers avoid these IF they are worried about legal stuff, and look for 'unrestricted use' or 'commercial use OK'.
But, since you are making a free game, as long as you aren't using it to market another product (game, etc.), in my mind that falls under personal use. Lots of people share art on the web via forums like this one, and in my mind as long as such art is not being used to generate income or promote other games, you should be OK using this.
Of course, some of the 'personal use' only stuff is licensed as such because the item in question is recreating another property (say a character from Supergirl, Robotech, etc. you get the idea), so with those, yeah that falls into the murky world of fair use... you are usually better off just avoiding those, but some people will still go there. It's a bit of a minefield though,... you never know when someone will invoke DMCA or something.
Do keep in mind that, even though you may only share your game or comic here, other sites such as SVS will often grab stuff here and offer it on their sites for download. So that's a double edged sword. You get noticed by more people, but then you've increased the chance of getting noticed by the legal types. And you have pretty much no control over that. Those other sites grab whatever they think is interesting, so that they can increase the traffic on their own sites. Hence, why avoiding the 'fair use' question by not using things based on other people's intellectual property, etc. is relevant.
No doubt this is one reason why we continue to see so many Simpsons based sex games ending up with abandoned tags around here. But I digress.
Anyways, something to think about.
Back to your question:
As to releasing free games, several people here do release free games, and it's a good way to get your feet wet in the world of adult game design, and maybe to learn new things like 3D rendering, Renpy coding, etc..
Others have already covered the 'assets acquired via other means' thing. This being a buccaneer forum and all, yeah that is done a bit around here, but you are rolling your dice and taking your chances. The odds may be very low that someone will notice, but low is not zero...
There are a few freebies threads here on F95Zone that can help you steer clear of that if that's a concern for you though. There is a LOT of free stuff out there that you can legally acquire and use to tell the story you want to tell/make your game.
Best of luck! This community is in general very supportive of new adult game designers (we do have a few grumpy people of course, all forums do), so don't be afraid to ask 'stupid questions'. DO check out the sticky threads on this subject in various parts of this forum first, though. It may save you some time, as they may have a lot of the answers you are looking for.