Are they really having less sex?
Most statistics don't register if people have sex or not, but if they are single or married. Starting there, boomers, and some stuck up Gen X, assume that "single" people means people who don't have a partner, and conclude that most Gen Y and Gen Z don't have sex.
But the fact is that marriage is declining since a really long time, what doesn't imply that the same apply to couples. Younger generation take more times before they marry, if even they decide to marry; they acknowledge the constraints that come with it, and the fact that it's an engagement that you don't take lightly. But this doesn't mean that they aren't in a long term relationship, and therefore that they don't have sex.