The pill and IVF are two big causes. Allows them to "remain unattached" for so long that when even nature says "hey you ducked around for too long", science can say "it's cool bruh, we gotchu".

If men had the same option of say based surrogate mothers, or vat grown babies, I guarantee men would be out of relationships far more. (not say ng this is better, both aspects ruin the inter human nature)

They claim its "just easier" to harvest and bank sperm than it is eggs, but this is just another gynocentric aspect of modern biological science.

It's also how third world nations can also have dying birth rates, it won't all be down to men choosing to opt out, but also a combination of antipregnancy aids and peak population already thanks to longer human life.

So we have large populations in small areas, delayed pregnancies, extended lives (and of those extended lives, many are unproductive/pointless regards birth but still consume the same resources). Birth rates fall because of science.