wow. my xh moved in with his girlfriend before they got engaged and my kids knew that they were sharing a room and bed, but nothing more. Since then they've gotten married and even if my kids do walk in, they're married. my own bf has never spent the night when the kids are home so they've been exposed to the double standard and are seemingly ok with it. They see mom get dressed and go on a date with her boyfriend and come home alone. My little girl prefaces having babies with after you get married.
I think you can still teach/share your values with your child in spite of what he's been exposed to...people have different value systems and this is how you do things in your house. You don't even have to address what he saw at mom's. Kissing the cousin...there's nothing wrong with that at all...many kids experiment with kissing, etc. However, if your child starts to act out on the other activities he's seen, I would consult a (sexual abuse) child therapist to evaluate any long term dammage. Some stuff is ok, but you'll know something is really off if he's doing other stuff.
Give you X some points for having talked to you instead of trying to hide this really big thing from you.
Love 
