This is a very, very difficult situation for a sensitive person like yourself. You are bound by love and concern, but you also know in your very bones that this isn’t a healthy situation. Unfortunately, you are now caught in a co-dependent web. It is unlikely that you will be able to establish boundaries without incurring anger, disappointment and blame from your folks. They have each gotten accustomed to turning to you instead of each other. It may well be that things will be rough as you do what you need to do to become an independent adult.
For your family to be healthy, appropriate boundaries need to be re-established. Your folks need to relearn how to be a couple. You need to get out of their marriage and into your own life. That means refusing to talk to them about their marriage and especially their sex life. There’s no reason to argue about it. Simply tell them you will leave the conversation as soon as it turns to anything about them as a couple. Then quietly just do it.
Becoming independent means increasing your time away from home without explanations or excuses. A your age, you don’t owe them a reason for your absence. Get out of the house, even if it is just to go for walks or to the mall. Even better, start calling up your friends an arranging for normal social get-togethers that take you out regularly. It means getting a job, a place of your own and an age-appropriate life style.
You don’t need to do any of this with anger or accusations or debates. Keep the conversations about it to an absolute minimum. If invited to defend your choices, remind them you are 22 and that you are working on becoming the independent adult you know they want you to be. Reassure them that you are not in trouble or involved in anything illegal. Lovingly steer conversations to more appropriate and neutral topics like what’s in the news, on TV or in an interesting article you’ve read.
It’s highly likely that you and your parents will have a very hard time with normal separation. Your parents each need help you can’t provide. You all need help freeing you to be your own person.
I strongly urge you to get in touch with a family therapist to help you in the project. If your folks won’t go, go yourself. Often other people in a family will eventually agree to treatment when one member starts to go regularly and starts reporting positive results.
I wish you well.