My mum and dad have been married for 39 years, both atheists, definitely had sex before marriage. They were both 22 when they got married, straight out of university. They knew eachother 3ish years before getting married. Quite a few of their compatriots did the same, some are still married, some aren't.
My PILs have been married for 40 years, both extremely devout Catholics, definitely did not have sex before marriage. They were 31 and 28 when they got married, quite late, by the standards of the time. They only knew eachother for 6 months before marrying which seems astonishing now.
Some 'young' Christian marriages are destined to fail. Some 'young' atheist marriages are destined to fail. Some marriages are going to be long and happy, whatever the faith or otherwise of their participants.
I think its partly a sex thing and partly the fact that marriage is seen as the bedrock of the community by Christians which leads them into taking that decision earlier than others. It isn't for me, either the Christianity or the marrying young, but I wouldn't criticise those who chose that path unless I felt they were entering into an abusive relationship.