Why are women ashamed of being seen in their underwear,

I am not sure if has more to do with the underwear, as opposed to the situation. Sure, a lot of underwear can be see through, which can affect things, but if at the beach, being seen in underwear vs bikini isn't a huge deal, at least to me. I will say, underwear isn't really made for getting submerged in water, so it can get more see through as well. I have been in situations at a pool where swimming in underwear did not make me uncomfortable. Now, if someone walks in on me in my underwear then yes I might be uncomfortable, but this is because I am not in a situation where I am expecting to be seen in minimal clothes, and not because it is specifically undewear vs a bikini that I am wearing. I am not sure if that makes sense or not.