Refuse

The price is too high. You'd be protected from getting pregnant by other monsters, but only because you'd already be pregnant by that one.

Of course, that decision means you need to move on. You leave the cave quickly, before it can decide that your continued presence signals acceptance of its offer.

Now you have to decide on a direction. Following the river in either direction means it would be easy to find your way back here. You could head away from the river, taking that risk. You could even head back towards your now abandoned village, though if they have people watching that will just lead to meeting the same fate as everyone else.