"Ctrl-F Marriage" gives no results. Surely decrease in marriages is the biggest driver. It's a lot easier to have sex with someone if you live with them. A straight person isn't going to find multiple new partners to sleep with per week and even if you have a boyfriend/girlfriend, unless you are cohabitating you probably only see them a few days per week.
The median American has ~5 sexual partners. So unless they're having a lot of sex with those partners when they're dating, marriage is still the biggest driver. I would bet the median person has had sex more often with their spouse than with all other partners combined.
Perhaps because the stereotype is wrong? Or rather, it depends on the relationship you have with your partner. Intimacy will mirror the rest of your relationship.
