I also wonder how much effect an abundance of other options to entertain yourself in general has on the frequency of sex.
For Germany there are statistics that children and teenagers go out less and have smaller friend groups for various reasons. One discussed reason is a shift to favour solitary activities at home (video games, YouTube, Netflix, etc.).
I don’t see why the same wouldn’t be true for young adults and adults as well. And then they turn on Tinder and hope that somehow a sex partner falls out of the clouds and doesn’t mind their untrained social skills.