When you've been with the same person for a while, things can get kind of boring. Many people, when asked how to spice things up, can only thing of a few variations: lights on/off, under the covers/on top of the covers, moving to the kitchen instead of the bedroom.
This might surprise you, coming from someone who writes erotica, but sometimes sex isn't what's sexy.
Some people have sex only after marriage. Others will do it in a committed relationship, with or without marriage, and still others will sling their clothes all over the room for just about anyone they meet.
And there's nothing wrong with any of those people