I'm a novice, but from the research I've done, the evidence is far from conclusive. There is growing evidence of the negative impact of microplastics on human health in a number of ways, but it is a massive leap to claim it is the primary cause for the phenomena you're describing.

In fact, the biggest contribution to declining birth rates is people have fewer children, not men being incapable of having children. And there are plenty of great sociological explanations for that. Changing gender roles, economic mobility, access to birth control, etc.

Edit: As somebody else said, it's a birthrate crisis, not a fertility crisis. "Fertility" is a loaded and inaccurate framing.

PSS: Even crisis is loaded. It just leads to people channeling their existing personal insecurities into large scale social phenomena.