Cowardice will put you and your opponent into the correct position. If you don't have it unlocked, train Submission with Alice.
Apart from that, the girl you're fighting needs to have and be wearing a strapon. If you strip them fully and they don't put one on themselves, it's probably because they don't have one yet. I know that Cassie never used to buy one, but she also has a scene and CG for it anyways, so I don't know if that's been changed or not.
Eve is an exception for obvious reasons, I think she just has a minimum arousal requirement like the player. In any case you will still want her in a dominant stance behind you.