I don't think people have sex or not because the cost of living. Poor people have had plenty of sex in the past. I think modern society is more occupying and people just interact less in a romantic sense because they are getting attention drained, in the same way a vampire drains your blood. We relieved the hard labor from peoples life but we also replaced it with attention suction devices that occupied peoples hours at much higher rates.

If you were a peasant in 1300s you worked hard probably 8-12 hours a day but once it was dark or the winter what you did radically changed because you couldn't see. It was just hanging out with people or going to bed, or other indoor task that usually involved other people or being around a limited light source. Those were the options of which one is kind of boring after a while. You'd thus interact with people and by interacting you'd end up having sex because it was fun when compared with doing anything else. Porn also didn't really exist so that as an outlet was basically zero so I'd attribute that to part of the attention occupying culture we have today