The BBC reports that sperm quality continues to drop. Specifically, researchers "found a 52.4% decline in sperm concentration, and a 59.3% decline in total sperm count in men from North America, Europe, Australia and New Zealand." While alarmist publications tout this as a 60% drop, the decline is accelerating and the researchers are concerned that inaction may lead to species extinction despite the effect not being observed in regions with high machismo, such as South America, Asia and Africa.
The study "aggregates 185 studies between 1973 and 2011, one of the largest ever undertaken." It supposedly overcomes selection bias occurring from patients attending fertility (virility?) clinics and selection bias of null results not being published in journals (churnals?). My intuition is that insights can be gained from studying transsexualism. Practitioners claim patients increase at the rate of 15% per year (doubling every five years), over many decades and with no end in sight. This is akin to Moore's law, Kryder's law, Butters' law, Hendy's law, Rider's law, Carlson's law or any other exponential halving or doubling. So, it doesn't take a genius to understand that it will become an increasingly widespread issue.
Regardless, masculine medical problems are vastly under-represented. By some estimates, spending on male medical problems is about 1/4 of spending on female medical problems. For example, when a man seeks help for a legitimate medical issue, such as declining testosterone, a patient at the lower end of the "normal" range may be denied treatment even if he is constantly exhausted.
Well, take care of yourself. Eat properly. Drink properly. Rest properly. Stay active. And if healthy food and exercise won't fix accumulated problems, consider hormone replacement. You may also want to watch two films which seem to be mentioned with increasing frequency and seem to predict our era with some accuracy: Children Of Men and Colossus: The Forbin Project. Children Of Men is the second bleakest film I've ever seen and the film I've seen most during its initial cinema release. It explores the scenario of global infertility leading to economic collapse. In addition to a nexus of cast and crew, the seamless plot and astounding compositing, the film is a fantastic example of mise-en-scène which is best explained by example.
Anyhow, enjoy the films and get your medical problems addressed.
Disclosure: People in my family are affected by virility and hormone problems. I have a professional interest in film, media encoding and art education.
Indeed, and referring to your '..common ingredients in food' another good guess would be phytoestrogens.
The problem is, as you state, '..there are an incredible number of possible culprits', this, compounded with the fact that there are an indeed an incredible number of powerful vested commercial interests associated with the continued usage of said culprits. Good luck trying to find out what 'They' know about it.
..and you'll upset a large portion of the population that wants to believe that LGBT stuff is genetic or a normal choice or an evil sin. Suddenly you turn all that into a class-action injury lawsuit for birth defects.
You might think that, I couldn't possibly comment...(on a particularly tin foilish sort of day, I often wonder if somehow it's all planned, a means to get rid of the 'breeders'...)
Flaxseed has 3x the phytoestrogens of soy. Guess what "healthy" ingredient goes into a lot of kids' cereals!
My guess is not so much the soy protein products (not terribly prevalent anymore) as the switch in the 1960s from lard to soybean oil, which now pervades western cooking.
Phytoestrogen is also a thyroid inhibitor, and even a slight drop in thyroid efficiency can lead to persistent, uncontrollable weight gain (exercise and calorie control will NOT fix this).
As to the notion that orientals eat a lot of soy ... to my understanding, only as a condiment, not as a major ingredient.