Dear Mary —
Yes, it is.
Sexual intimacy, that is, becoming one-flesh, is meant to consummate and renew the covenant made between spouses.
Sex outside marriage, whether it be with the young or the old, does not renew a covenant commitment hence, the act is not one of love, but of people using (actually abusing) each other for pleasure.
The essence of true love is permanent commitment, not emotional or physical desire.