What would happen if men's fertility was limited like ours?

Hypothetical scenario:
- A strange mutation took place and spread through all countries, globally. It's a permanent change and nothing can be done to reverse it. Suddenly men have a limited reproductive cycle to father a child. Their sperm count will drop drastically after age 35 and the possibility of fathering a sperm is much lower. After 40 it gets hard to nearly impossible.

What would possibly happen if men had a limited time to have kids? If that wasn't in their favor anymore, what changes do you think would take place?
What would happen if men's fertility was limited like ours?
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