For one, unlike most mammals, humans don't have fur that can keep them warm

Well I don't think most animals have to worry about their outfit being safe for work since they're all unemployed freeloaders

So when flowers rub their gametes on passing animals in the hopes that those animals transmit them to other flowers, it's fine, but when I do it, everyone loses their minds???

I believe there's a good number of tribes near the equator that don't wear much, so not sure it's even universally NSFW, esp. if your job is a hunter/gatherer. But people wear clothes to protect from sun/pests/plants also. Would imagine we started wearing clothes as soon as we learned how, as it has many benefits, and then the sexualization came as a result of no longer seeing all the parts, and being curious.

Human have covered themselves from the elements basically as long as it was possible for them to do so

humans must’ve walked around naked before covering up for some period of time, right?

Some still do, but they tend to wear some kind of genitalia covering.

Contemporary humans have disproportionately large penises compared to many other animals. Perhaps when we started evolving larger penises we co-evolved small dick jokes ("I was in the pond!"), leading to habitual use of various types of penis-hiding clothing.

Hermit crabs have entered the chat.

The change happened when we lost all-over body hair.

For me the problem is, with a few exceptions, humans are pretty ugly when naked. I prefer them in clothes.

I mean nude beaches are a thing...

The earth was in an ice age around 19,000 - 26,000 years ago - perhaps it started around then when everything was quite a bit cooler.