Earlier quoted context omitted.
Weight loss and proper exercise will probably deliver 99% of the improvements for 99% of the people. Stressing about heat, radiation, micro plastics, or other random stuff seems like extra stress for not that much benefit. Most people in Western world are either out of shape, or straight up obese. It makes sense that, that will have huge negative effects on fertility.
In some communities, weight gain is a down-stream effect of "metabolic syndrome", and the "solution" isn't to count calories or exercise more - it's to simply eat _differently_. Sugar is a particularly odious contributor to problems. Like OP, my wife and I also struggled with infertility for a few years (two miscarriages, years of doing everything "right", and not getting pregnant.) We're finally pregnant, and out of…
I had the exact same experience with doctors just saying there was nothing a man can do to improve fertility. “It’s just genetic”.
I also second the case against sugar. I should have called out refined sugar specifically in my list. That was one thing I cut out 100% even in ingredients lists (this is tough. It’s in EVERYTHING. Even loads of savoury things that have zero business having sugar in).
Side pondering - I’ve often wondered if McDonalds got a really raw deal from Super Size Me (great documentary) and that it was just the super size soft drinks that were the cause of problems - remember the guy in there who has eaten thousands of Big Macs? But he never had the drink. And he was thin as a rake (dunno how many kids he had though!)