If this what you meant to say, you should rethink your argument.
Do you have a problem with nudity, generally? Can you articulate your problem? There are valid reasons to dislike nudity, but I really hope your reasons aren't as sexist as you've made them sound.
Are their valid reasons? Or just reason your relative moral standard would agree with? Is sexism even bad? Who said it was? How do you know they're right? Is cannibalism wrong? according to what objective moral, universally true standard? Did god tell you? But who told god? Where'd he find this universal truth? Or did god just say it was so....in which case, does that just means we judge standards relative to god's subjective, relative view?
Who said anything about objectivity?
I'm asking that he present a valid non-sexist argument against nudity. That doesn't mean I'll agree to it.
I'll also remind you that I'm an atheist.
Sexism is wrong because it derives from the assumption that there are differences between men and women that are fundamental biological realities without adequate justification. It's not sexist to say that women are the only sex who can give birth. It is sexist to say that men are the only sex who can do science.
Dude, I was just poking fun at the idea that the idea that something is "right" or "wrong" is entirely made up concept that at it's most basic form evolved from social contract theory so society could sustain itself. Eh, that probably makes me a bad Christian to say that.
Whether you're atheist or not doesn't really matter. Whether he's sexist or not doesn't matter. Whether I believe new born babies are delicious snacks when lightly fried and tossed with Soy Sauce doesn't matter.
This statement: "Sexism is wrong," fundamentally, relies on the fact that we must have a common understanding of what's wrong. Or one of us must have a better understanding. Or one of us must be mistaken in our understanding. Is it wrong to make a false statement? Who can say. Maybe it's just weird.
You're literally arguing an opinion is what it can be boiled down to.
As for the literal version of wrong of saying something like: The sky is green when it obviously doesn't look like it is. Or that only the male sex can do science. <- We must then commonly define science. Then we must define what is a male. And I can be more asinine and say we must define what one means by "only" or "can do." You might eventually catch me in saying a false statement and disproving it with empirical evidence, but then you have to prove that it's "wrong"to make said false statement and I can stone wall you on that all day for any reason I can imagine.
I'm obviously just poking at you and others, but I do find it a bit amusing in a dark way when I remember that ultimately none of this means anything anyway and peoples individual sensibilities mean very little in the grand scheme. And they are individual since no two people think exactly alike. Person A and Person B might have very similar moral standards but dig deep enough and you'll inevitably find difference.
EDIT:
Also, also, there's plenty of arguments against nudity.
Comfort
Protection
Awkwardness from body image and mating psychology on both sides. Men will check out women and women will check out men and everybody will check out everyone. Can't change cultural brainwashing of what's sexual :P and it's also inherent in biology.
Competitiveness creating power dynamics and social hierarchy
Problems inherent in clothing options/choice and/or lack there of came from culture not from the clothing such that nudity will create it's own cultural issues no doubt. and there's also the real issues that come from biological impulses and psychology tied to it and the the practical reasons against nudity such as cold weather and comfort levels.
