Is infertility most-commonly (or even often) caused by promiscuity?

As a primary cause, not directly, but it does mention it as a top cause of tubal issues, which is related to infertility, and as some increase factors for infertility /end of page).

I do believe that, because the presence of STD overall as a proportion of sexually active people has diminished/has treatment, the effect they may have in fertility has decreased.

I’d have thought abstinence would be a leading cause of infertility.

Statement made by the elderly are commonly rooted in outmoded knowledge, and may also be characterized by antediluvian-formed dementia.

So no - it’s BS.

I would think that promiscuity would cause more pregnancies than it prevented.

I disagree with the reasoning: the particular statement happens to be bullshit, but not because it was said by someone who’s old.

Infertility is MOST commonly caused by age. That should go without saying. many other things can contribute (including smoking), but nothing reduces fertility like the passage of time.

Only to the extent that it’s so much fun you might forget to have kids until you’re 40.

There’s a lot of beating on this old doctor mentioned by the OP as if he’s some demented old man, but he’s generally correct if you take PCOS off the table which is an inherent physiological/genetic issue. For things you can do something about re behavior having lots of sex partners and the higher risk of STDs associated with that behavior is the # 1 risk for infertility.

The statement is OBVIOUSLY bullshit, even to the layman. I was simply offering tangible reasons why that might be so.