There are two reasons.
The silly reason is that my company makes at least one sex toy.
The real reason is that it centres "work" as the default state, that sexuality is something to be done while you're not otherwise busy being productive for capital.
Yes, porn is one of several things you probably shouldn't be looking at in the workplace. But you also might not want to see smut because you're with your family, on the bus, sex repulsed, or even just not in the mood!
And that's to say nothing of the way it centres a very particular kind of "work." We all know sex work is real work, but it also erases the people who have to moderate the worst Facebook and Google have for pennies on the dollar, or paint tasteful nudes, or any of a hundred other sex-adjacent trades.
We will never unravel Tech's creeping puritanism with a binary like SFW/NSFW. It's the kind of thing that gets nursing women harassed and idiotic phrases like "female presenting nipples."
I am glad we are all realising that we work so that we may live, not the other way around. Now let's stop defining sex as "when you're not otherwise busy."