As a married man with a kid, there is something more to long term romantic relationships than sex and reproduction. There is a joining of your lives; since my now-wife moved in, my life centers on two people instead of one. I don't make all decisions about what I will do and what my life will be like on my own. There is both a loss of control and a comfort in this. Where are we going to live? How are we going to spend our weekends? How will we decorate? All of those things used to be my decision on my own; now, the two of us choose together.