I do not believe it is immoral to session with someone who is of legal age, unless that person is under duress of some kind. One of my friends used to pick up street walkers and negotiate their price down to peanuts. He knew they were drug users and desperate for their next fix. He said getting their price down to an embarrassingly low amount was half the fun. That, I find wrong. I lost a lot of respect for him over that. But, a healthy, sound of mind person doing exactly what they want to do, I don't find fault with that.