How are yoga pants any more sexy than any other pants? Because you can see the outline of her bum? Can't you see that in any other type of pants as well?
It's skirts and dresses that are actually immodest what with ladies' private parts uncovered and anybody able to take an upskirt shot. As my husband says when I wear a skirt ( nearly never), "Easy access!"
Clothing "modesty" is a social construct and not objectively true at all.