Pack her One Piece

My daughter is going to lad & lassie camp tomorrow.
Honestly, I have no clue what it is.
I do know that my momma signed her up for it, though.
So earlier this evening, when I was doing laundry my mom told me Xiaden would need her one piece.

Honestly, I don't know where this one piece came from. Probably in a welcome box of hand-me-downs. I put her in it when I can't find her (very tasteful) tankini.
But something struck a cord in me, and my poor mother got the backlash.

Why does my daughter need to wear a one piece?
So I ask my mom, who simply states that it's the rules.
But why? Do the boys have to wear shirts?
Obviously, she didn't know, because Kamden isn't old enough to go to Lad & Lassie camp, so she paid no attention to the boys' rules.
But I, being the difficult child my mother loves and tolerates, am incensed.
My mom tells me its fine, just send her in the two piece & a shirt.

My six year old girl just had a Jurassic World birthday party two weeks ago. I just bought her a dinosaur backpack. Something I knew would raise eyebrows because it's seen as a "boy" thing. Even Kamden tried to tell me it was a "boy" thing. I don't know who taught Kamden the entire "boy" and "girl" things were recognized, but that irked me at the time too. So I calmly explained to him that he can keep playing with the My Little Ponies & watching Princess Sophia even though everyone else thinks they're "girl" things, just like Xiaden can love velociraptors, even though they're "boy" things.
And it was as simple as that.

The only time I told my son anything was a "girl" thing is when he wants to wear bright red lipstick somewhere in public. I let him wear it at home, because he just likes to leave his kissy prints on everyone's cheeks. Just like Xiaden & Rhyssie. But when we go into a grocery store & I'm rocking my red lips, I will either put chapstick on him, or I will kiss his cheek so he's still sporting the red.

That's the only time.

Because I want all of my children to know that they're equal. They're all able to do and like whatever they want.

By telling me to cover up my daughter's bare midriff, you are telling me to hide the swim suit she loves. You are telling me to teach my daughter that she can't show her belly, that it's secret.
By telling me to do this, you are sexualizing a 6 year old girl's body. By telling me to do this, you are showing her that the boys don't have to wear shirts, but she does. By telling me to do this, you are showing my baby girl that her body is inappropriate.
This is where sexism starts. In Kindergarten, apparently.

Other Examples can be found here, here & on google.
I know I sound crazy.
I know they're just the rules.
but I want to know why, precisely, they are the rules.