You don’t need science fiction to imagine this; simply look at societies that just got through a brutal war that cost them a lot of their young men, although I suppose you could watch Dr. Strangelove for some inspiration.

Raising children is a lot of work. It’s twice as much work without a partner, not to mention not having the extra income. (I don’t imagine men in a 2:1 society will be interested in paying much child support.)

The number of children in advanced societies (particularly in cities) has already plummeted. And I don’t think many women are going to want to sign up to be single mums.

As far as sperm selection goes… that’s already an option popular in China and India, along with sex selective abortions. The preference is for males, though, for various reasons. I am not convinced the end result of this is wholesome.