Lovechess : Age of Egypt
They just released a demo of the new Lovechess version, the demo of the Greek era was downloaded at least 3 million times. Be sure to set the graphics to the proper resolution, to experience the best graphical quality.
Lovechess game demo
The demo of the "classic" Lovechess can be found here:
Lovechess the Greek Era

Virtual Hottie 2 demo
This virtual sex simulation uses Mocap for more natural animations, the demo has some limitations, but gives a decent impression of the game as a whole.
Virtual Hottie 2 Adult game

3D Sex games ( Sex Villa )
A game with tons of features and still one of the most downloaded demos out there, although the game is around for a few years.
They also have a gay version, the only gay computer game in existence, if I'm informed correctly. Both the straight and gay version have a demo to give it a try.
3D Sex Games website
The Gay version of the game

Second Life
As mentioned this is not a real demo but a free trial account, the only limitation is no money to spend inside the game world. But there are still many things to explore, even without cash.
Second Life

3D Plaything (what's in a name )
This game was labeled Somasex for a while, but they changed it's name to 3d Plaything recently. This site offers some other things besides their main game, for members, like free galleries and smaller card games.
The demo is available from here:
3D Plaything

Active Dolls
A cartoon style game, with girl creation, photography sessions and "girl on girl" sex, the game's style is something you like, or hate. The game demo gives a good idea about the game, so easy to try if it's something you fancy.
Active Dolls demo

Play Sex games (.com)
This demo runs in your browser window, is you have Virtools 3D Web Player installed it will run directly, or the installer will ask you to download it. The game modelling work is the main drawback in my opinion, it does have quite a lot of features though.
Play sexgames.com

EGirl demo
A stripping and "play with your girl" game, the game scenes look good, the models are detailed, worth a try.
EGirl

If you know an adult game demo that's not listed here, please let me know, I'll add it as soon as possible.