Or it's simply because young people spend ever-increasing periods of time living in cramped shared accommodation or with their parents, due to the dysfunctional

Or it's simply because young people spend ever-increasing periods of time living in cramped shared accommodation or with their parents, due to the dysfunctional rental market in most Western Anglosphere countries. This certainly isn't conducive to sex, and often makes it difficult or impossible to host friends (meaning that people often choose to meet at venues such as restaurants instead, a situation which often necessitates more infrequent catch-ups due to the expenses involved).