The PS2 has an impressive catalogue of not only "regular games", but also PC gal games (think Japanese dating sims) ported over with the adult scenes edited out.
Called "visual novels", these games are more reading intensive than anything else. There is a small, yet rabid, market for them; however, PS3 hasn't exactly been fertile ground for visual novels.
A recent Japanese magazine article on "gal games" points out three reasons why these games aren't coming to the PS3:
1. Titles need to sell 30,000 copies. 2. The games need online capabilities. 3. Graphics need to high-def.
Stumbling blocks for developers accustomed to easy PS2 ports. Granted, these rules aren't written in stone, and erotic tentacle game Tears to Tiara has proved there is a market for these types of games on the PS3. As the console install base grows, we expect the number of these erotic-tinged games to get bigger.