I´d also recommend you to get tentacles as those will often open up for scenes where you can stick it in several holes.
If you´re really desperate just browse through the wiki. You can find several characters with butt stuff scenes, just by searching "anal".
But personally speaking, discovering as you play is the best way to do it. You might find it more fun that way, and the wiki doesn´t always mention the specific types of sex scenes a character has.