Cardinal Vincent Nichols says casual sex could be with someone's future husband or wife, and that it makes it harder for couples to stay together for a long time, particularly through difficult periods.
He added that this was a great shame when most young people want to have permanent relationships and stable families.
Cardinal Nichols told the Daily Mail: "Nobody wants a husband or wife who is unfaithful.
"I don't doubt that most young people aspire to having their own family.
"So what we have to get across to people is that casual relationships before marriage is actually being casual with somebody's future husband or wife."
Research from the Marriage Foundation earlier this year found that, while just under one in five unmarried couples who are living together are parents, they account for half of all family breakdowns in the UK.
Cardinal Nichols is currently in Rome for Pope Francis' Extraordinary Synod on the Family.