Can my employer force me to wear a bra?

First of all - wearing a bra is NOT in the dress code, which I signed upon employment.

I work in an office and adhere to the dress code, however another female employee recently complained to my boss's wife (who is like the boss) and she wants me to wear one, but I heard about this from another employee who is like her assistant. (So no direct order from boss yet)

Now, I have very small breasts. A-cup sorta. Many men have larger breasts than I do. I do NOT need a bra for support and since I stopped wearing one about 8 months ago, my breasts have felt FANTASTIC - soreness is practically GONE. I've also been reading about how bra-free is better for your breast health... we have muscles, ligaments in our breasts that atrophy (get weaker, like arm muscles in a cast for a long time) when not used, which causes our breasts to sag more... so they actually stay perkier longer withOUT the bra. Not to mention, my breasts used to be sore all the time and felt super restricted with that unnecessary bra.

SO, you can sometimes sort of see my nipple shape.

HOWEVER, I can see every single man's nipple through his shirt in this office almost every day.

Sounds like sexism to me.

Small boobs, nipple is the same as a man's who is not required to wear a bra or anything else to hide it... so can my employer force me? Can I use sexism as a way to back this up?

I don't want to go back, going braless was one of the best decisions I ever made.