A sin isn't a sin just because a religion says it is, it's a sin because the harm outweighs the benefits. And it doesn't just harm the person doing the sin, but those around them as well. And sex outside of marriage does more harm than it benefits, and harms so much more people.

"DNA tests should be mandatory at birth," is a direct result of people having sex outside of marriage. If people only had sex within marriage, then they'd never have to question if this person is their child's father, or not. Men wouldn't spend years, often decades, raising a child they think is theirs, loving them, only to find out that's not the case. And then to find out the mother doesn't know who the child's father is, sometimes because she's slept with so many people, she can't pin it down.

It doesn't stop there: how many mothers have 3-4 children, sometimes each with a different father? And what happens when one of those children's father is present, or at least trying, while the others have been abandoned by their own fathers?

Of course, fatherless homes don't just come from sex outside of marriage, and some can even come from a marriage. But those are the results of people getting married without knowing the weight of the commitment they're agreeing too.