Vacuum Behavior.

It's very similar to the way that ANYONE wants to have sex even if it's not to procreate. I have sex all the time, but never once with the intention of procreating. Sex feels good, because people who enjoy sex tend to have more children than those who don't. After enough generations, almost everyone who is left enjoys sex.

In short, wanting to have sex even after you can no longer have children doesn't have any DISADVANTAGE.