You can be serious and casual at the same time. Just because you do more serious stuff doesn't mean you have to stop doing the sex parties and just having fun, it doesn't even necessarily mean you have to stop sleeping with others.

Try and let the things you want to change change, and let the things you don't want to change stay the same. It sounds too good to be true, but in my experience you have to try for it if you want things to last.