I'm 25. Whenever I have sex with my girlfriend (20) I fantasize that it's her as a 16/17 year old. Is this wrong?

Your teenage wife (other question) doesn't mind that you have a gf?

As a child who was abused and an adult who was raped, I find you to be a very sick individual.

its much better than pretending you are porking some one else

Technically it is very illegal for an adult to portray to fantasize and that they're having sex with someone under the age of 18 years old. It's technically called sexual exploitation of a minor. I've got this rule if you're not 21 years of age and you can prove it with the driver's license I'm not talking to you about anything sexual.

That is wrong. That's very wrong, actually. 

You have a sexual attraction towards teenage girls and that's called Ephebophilia. I don't know your girlfriend but I think she would be more weirded out than upset.

About how wrong it is, well she's an adult so you doing it with her is not bad I guess? If you tell her, she might not have a good reaction but if you don't, won't you feel guilty? It feels kinda wrong tbh so I suggest you tell her. Maybe she would be fine by it who knows.

As long as you don't project your desires on an actual teens. It's wrong and illegal. 

Seek professional help if it gets out of hand.

Fantasies are healthy, and they're meant in this case to be private. As long as you don't act upon your thoughts and it enhances things for your girlfriend that's fine.

i dont think so