If it could be established that some men are more chemically resistant than others, then it could be natural selection, too. Though, as got repeatedly drummed into me, evolution is often not about survival of the fittest, but the luckiest....
My degree included evolutionary biology. Quite right on that last, especially where a mutation-based change happens to be more adaptive to a current environmental factor.
It's also unfortunate that the old Darwinian nomenclature, "fitness, " is often confused with its meaning in the popular sense. Fitness, in evolutionary theory, means nothing beyond adaptive phenotypes with respect to an environment where that phenotypes reproductive chances are greatest.
There are also quite random factors affecting allele frequency, like genetic drift. But that's for another chat.
In the case of endocrine disruptors like PFAS, a set of alleles that happened to increase the body's resistance to its sterilizing effects would, in a natural setting (without advanced medical interventions), increase in frequency in the population, i. e. be selected for. Assuming that set of alleles didn't have some other maladaptive effect, too, like some inability to absorb key vitamins. We don't know yet, and we do know that the frequency of intercourse in hominin bonded pairs is such that even males with low sperm counts can sire multiple offspring. But, yes, as counts drop very low in large segments of a population, then males with more resistance to endocrine disruption would possibly start to pass on their genes more widely, especially in regions with less medical intervention availability to boost fertility. (but there are many confounding factors there...)
Poll
