It is sexist to treat men and women differently without a good reason, and it doesn’t seem that there’s a good reason in this case, which means that doing so is sexist.

Regardless of morals, from a biological perspective, treating male and female nipples differently is irrational, since male chests are also physical sexual characteristics. To be clear, I’m differentiating from the perspective of “these two body parts are shaped differently and therefore have different needs when fashioning clothing” perspective. There’s a good reason to do that (though there isn’t a good reason to enforce it). But from a “one is sexual, one is not, so one needs to be covered and the other doesn’t” perspective, what’s the reason? You indicated that they’re physical sexual characteristics but as I already pointed out, physical sexual characteristics aren’t generally required to be covered. People find both men’s and women’s chests (and other body parts) sexually attractive, so that isn’t it, either.

If you’d say that society treats them differently because we’ve historically objectified and sexualized women (and continue to do so) and as a result women’s bodies are considered sexual in a way that men’s aren’t, and this is ingrained in our culture at this point then I would agree with you. I just don’t agree that that’s a “good reason.”

Males can also lactate if they have elevated levels of prolactin (which can be genetic). It’s fascinating stuff, some cis guys can allegedly get “sympathy lactation” after their partners give birth. There are also quite a few other species of mammals that have males lactating over females.

https://www.scientificamerican.com/article/strange-but-true-males-can-lactate/

However, guys, if you do start spontaneously secreting fluid out of your nipples, I would suggest getting yourself to a doctor.