It wasn't a very common practice in earlier generations, it became an increasingly common practice in newer generations, and the shift could very easily produce unstudied consequences to health at scale (whether this or other things).
https://www.latimes.com/california/story/2023-08-03/young-ad...
For one, quite famous historical example see the Kama Sutra. It does not take more than a quick Google search to find a million other examples, eg https://en.m.wikipedia.org/wiki/Erotic_art_in_Pompeii_and_He...
Check out this AskHistorians thread. https://www.reddit.com/r/AskHistorians/comments/17zztg/how_c...
Because people have been getting it on quite creatively forever and because they were often writing about it (and because many older living people got around too), we can make reasonable -- if not authoritative -- suppositions about the specific things that people were getting up to in various communities at various times
But more seriously it's a gram negative bacteria so it has a polysaccharide armor that could resist a passage through the digestive system.
There are also many much older literary sources on practice and technique that are quite rich and detailed but don't really give it much attention.
You're correct that none of that can provide authoritative counter-evidence to the claim that it's always been as popular and widespread as it is today, but given that many practices do come in and out of fashion, it's easier for most to assume that the particular quiet of the oral and literary historical record about this is because it wasn't popular than that it is the one secret thing that nobody blabbed about in topical literature or ran across much in their own experience. I didn't even think it's recent, dramatic rise in popularity was contentious until you pushed back on it just now.
I'd personally put the burden of proof on demonstrating that it was similarly common rather than that it wasn't. But I would understand those determined to disagree.
There are cultural trends and fashions in intimate practice, though, whether or not you accept that this is one of them.
...followed by...
> I mean, I can't speak for what was unmentionably hot in the 19th century or whatever
...seems to indicate you should not have been so confident in your initial assertion. A great number of things were likely less mention-able in prior generations, including the act of hetero-normative sex; I suspect people had plenty of sex then, considering we exist at all.
Surely not being openly discussed isn't evidence of it being extremely prevalent either. And given that, it's really hard to imagine reliable evidence one way or the other. But the person you're responding to did not make an obviously ridiculous statement. We all know it's not provable, but I'll happily give them the benefit of the doubt here.