"Crop top" and "sports bra" are not interchangeable. Some particular garments might be fairly described as both, but certainly not all.
Again, my concern is not for modesty or sexuality, it's about decorum. Boxer shorts aren't provocative, but they're inappropriate. Long underwear aren't provocative, but they're inappropriate. Undershirts aren't provocative, but they're inappropriate.
I'm not saying "sports bras are inappropriate because they're too sexy," I'm saying "sports bras are inappropriate because they're bras."