[Because a super-genius baby with mutant powers could very well bring about the apocalypse. Just one of many reasons.
That said, his resolve isn't as great as it was at the beginning of this. He may be thinking about statistics. Very specific statistics — like how the risk of pregnancy is around 15% depending on where in the cycle intercourse occurs and yes it only takes one time but the odds of avoiding that one time are... decent. Not great, not even good, but decent.
It's dangerous, this lack of blood. It makes him stupid, and after another moment he breaks the kiss to whisper against her lips.]
Kitty... [Breathless and raspy and desperate. They need to figure something out.]