I think the sleaziest game I've gotten into on steam is Gal Gun 2, which is actually really fun and well made. However, it's basically an on-rails shooter where the goal is to use your vacuum gun to suck the clothes off of random girls (also suck up little demons). There's also some mildly pervy minigames involving flirting with various girls. Overall, not the worst, but might be questionable to some people.

The most obscene game I've seen on Steam though is "Mutant Alley - Do the Dinosaur" which is a gay anthro dinosaur sex simulator. I've been curious about it and maybe I'll try it out once Steam implements these changes.