My own interpretation of this is that 'be fruitful and multiply' means have at least three kids to replace the couple. Let's say a man's wife's died and he had three kids already, and he was marrying a young woman who'd never had kids. Multiplying means having two more. Anything less is replacement, not multiplying. That's how I'd interpret it, personally.
Pragmatically, if a 23-year-old Christian womand didn't see it that way, wanted marriage and sex, but no more kids, one option would be to marry a man who couldn't have kids or marry a widower who'd had as many kids as he wanted and had a vasectomy with his first wife. That way she wouldn't have to have her tubes tied. If she changes her mind, reversing his would probably be a lot easier, medically, even if the chances aren't 100%. I read they'd come out with a more easily reversible method.
One of the possibilities for a 23-year-old to consider is that, as her biological drive clicks in, she might actually start to want to have children. If she changes her mind and she's married to a sterilized man or she sterilized herself.
The other issue is that birth control, condoms, even vasectomies, are not 100% foolproof methods of birth control. It's wrong to have your baby murdered in the womb, so anyone who gets married should be willing to be a parent if their reproductive activities actually lead to reproduction. The reason for the sex drive, after all, is that it drives people to reproduce.