Easy, you start treating her like a person. If she is in a relationship with your partner, she should be able to interact with him how they both want to. If they want to have sex, they should. They are girlfriend and boyfriend. Not your possessions. Let them have the relationship they want to. If you have a good husband and/or a good girlfriend who want to be with you, they'll maintain the relationship they have with.you too.
Why are you trying to.deny her the opportunity to meet te needs that you.don't even want to meet? Don't you feel.selfish and uncaring?