Responsibilities (partner, children, money issues etc) leave less time for hobbies. If you only have yourself to take care ofand earn a basic living there's more time available to pursue personal interests

as for sex, two things. everybody's sex drive seems to ne different in my opinion. one thing I noticed in relationships though. if women no longer enjoy having sex, it turns out they are disappointed in the way things are headed in a relationship and it is an early sign of the relationship ending, which still might be a while away