Well there’s a stretch of an analogy. Unless you’re willing to only hire married women (which probably isn’t legal), you’re going to have some single women teaching. And many of them will be sexually active, no matter what you have them sign. Many single people are sexually active without intending to get pregnant, no matter how wise that is. They take the risk that they might get pregnant, and sometimes they end up that way. Your position forces her to pick between her kid and the job. A new child, but no job, no spouse, and (since the firing would be for cause) no unemployment benefits. For many women, that would mean “goodbye kid”.
If you think signing a piece of paper is going to change her behavior once she’s employed, you’re kidding yourself. Yes, you’ll have a few less single teachers choosing to work at Catholic schools, but you’ll have a few more dead babies too. No thanks.