It's Called 'Underwear' For A Reason...

Okay, what's the deal with the way some women dress?
Ladies, please, help me out here. I seriously want to understand, if I can.

I stopped at a BJ's Wholesale Foods last night on my way home from work. I had gathered my purchases, and rolled up to one of the registers; I started unloading my cart onto the cashier's conveyor belt when I noticed the family at the register next to me. It was an extended family, what looked to be a woman of 18 or 20, her mother, her aunt, and her grandmother.There was also an older man, obviously the grandfather, and a very young boy who may have been the young woman's little brother, but could have been her son. I never found out.
I noticed them because they were pretty loud, and they were such a large group they kept spilling over into the line at my register. Especially the younger woman, who seemed to have a lot of energy and just couldn't stand still. She especially caught my eye.
Well, not her exactly. More her underwear. She was wearing white bikini briefs with a pattern of little hearts on them.
How do I know this?
Her underwear was clearly visible beneath the amazingly tightly stretched cotton fabric in the shorts of the one-piece jumper she was wearing. I think the jumper had started out as terrycloth, but terry is thick and in no way see-through. This terrycloth had been stretched so tight I think it was in danger of Total Reality Failure; one more ounce of pressure on this jumper and it would puff into non-existance and Reality would have to Reverse Engineer itself so that the cloth had never really existed.  The Emperor's New Clothes had been written about just this kind of stuff. It was stretched so tight I was afraid to bump her, with either myself or my cart; the slightest nick in the fabric and it would have burst like an over-filled balloon, leaving the poor girl wearing naught but a raggedy necklace while the rest of us picked little bits of partially imaginary cloth out of our hair and clothes.
And through this peach colored cloth that had been stretched to the very edge of self-destruction, much to my dismay, her underclothes were clearly visible.
Through all this I was looking a bit sideways at the other women in the group. Here were the mother, aunt, and especially the grandmother of this girl, and they knew you could see through her clothes too. They could see what I saw, and (this was the part I was having trouble with), they were okay with it! I send Handsome back to get different pants when he outgrows them because his socks are showing!
This is something I've seen before, and I didn't understand it any better then. Are there any women out there willing to take a stab at explaining it to me?
Am I just an old, disapproving fart?

Talk to you later!