Others have said some great things, so I'll just address a few specifics from your post:
- Shorts, of almost any length, are not generally appropriate for white-collar jobs. Shorts that end just above the knee are generally accepted.
- Pretty dresses in the office? Sure - but again, some sundresses (strapless or spaghetti-strap) or dresses that end mid-thigh are not generally appropriate for white-collar jobs.
It's a double standard that in many offices (including the one where I work currently) men can wear shorter shorts (mid-thigh) and no one raises an eyebrow, though most people who do wear shorts wear longer ones. I do not see any men in my office who ever wear tank tops, even on the hottest days.
It is possible to find clothing that you like, that doesn't require special care, that is more universally considered office appropriate for business-casual environments. I know because I have closetsful - and it's the kind of stuff that I wear on my days off, as well as during the work week. Depending on the climate where you are, woven-cotton skirts - put in the washer on delicate cycle, hang dry in the shower - or closer-cut maxi dresses - usually knit jersey, wash-and-wear - are perfectly fine in warmer weather. Capri pants with either a rayon or knit top are also perfectly appropriate, and while they're not as easy to find, there are capri pants made of thinner materials (so that they're not so stiflingly warm.) There is a midpoint between "teenage summer gear" and "suit-and-tie corporate uniform." You just have to find where, in that range, you personally are happiest.
When the weather is cooler, I wear jeans and tee shirts quite often. I have to wear shirts made for men, because I have broad shoulders and a large chest, so "womens-cut" tees are too tight for me to feel comfortable in. I frequently rock the jeans-and-tee look, or the jeans-and-knit-top look. In summer, I switch to lighter fabrics. I don't do that to "fit in", I do it because jeans and tee shirts are comfortable. So are palazzo pants and cotton blouses - which is an example of what I wear in the summer.
And yes, it does suck that men seem to be able to get away with things that women can't in STEM; and that while people say that STEM needs more women, it's become a more, not less, hostile field - the hostility is just less overt, so women don't find out about it until they're further into the field. But this is how humans are (anything different from Us is not fully trusted.) Changing minds and attitudes is a slow process, like water eroding rocks. Recognize that these attitudes exist, figure out how you want to react to it and why, follow through and move on.
(And to answer your question about why cuteness tends to equal sexiness, at least in America: that's what happens when a Puritan-based culture/protestant work ethic meets the rush of full-bore capitalism: anything can be packaged and sold, and thus generate profit; anything that distracts from productivity is Not Good; womens' sexuality is commoditized even more so than mens'; and so women looking at all attractive, let alone desirable, is viewed with disdain because it distracts from productivity. Yes, it's irritating. And stupid. And circular thinking. And yes, there are lots of people, including many men, who do not think this way. But that's a whole other rant for a whole other site, and I won't go there because it'll be a huge essay.)
Edit: judging by your edits to your original question, the comments you added after various answers, and the answer you accepted, it sounds like the question you may have really wanted to ask is not "is there a dress code for women in the software industry" but "why is there an unwritten dress code for women in the software industry" - which is both a different question from the one you originally posted, and is also a question that needs to be asked. (Also note that men-as-a-group are subject to unwritten dress codes, even if those unwritten rules are not always as narrow for men as they are for us. Again: that's the way humans are. Go find a workplace where your co-workers will not judge you negatively on non-performance-based issues, or dress/act as you will and develop the skill of disregarding those negative opinions and comments.)