On the original "guys writing girls" point:
I think it's worth remembering that people come to E for different reasons and want to get different things out of it. Elliquiy is primarily an adult roleplaying/erotic writing community; some people put a lot more emphasis on the adult/erotic sexy times, others on the roleplaying/writing. If what someone wants and what they enjoy is kink fulfillment then it's hardly surprising that they're not necessarily creating the most three dimensional, well rounded, realistic characters... the writer doesn't need to do that in order to get what they want out of their writing. Feeding into that, stereotypes exist for a reason and are a very popular (and frankly, useful) part of kink fulfillment. To take a simple example, the generic "NC story where a barbarian captures a high-born princess" story basically requires a brutish, crude, dominant barbarian and a sheltered, gentle princess to work. You can go beyond those limits and those stereotypes... but if you go too far from them you end up not telling the story you wanted to when you began. That's not necessarily a terrible thing; I fondly remember one of my stories here that started off as a generic NC story featuring a barbarian warlord sacking Rome and taking a priestess as a slave but which by the end had turned into a sort of buddy-cop, comedy tale... but it wasn't the story my partner and I originally wanted to write.
Just as people tell the stories they want to tell, people write the characters they want to write. If they want to spend hours creating them and put huge amounts of thought and detail into them then they will. If they simply want the character to quickly be at hand so they can get straight on to writing the stuff they really want to write then they will as well. As long as they and their partner are happy with that then there's not really an issue; despite being posted semi-publicly stories on E aren't really for public consumption.
On cross-gender play specifically, I've seen some male writers write fantastic female characters and some female characters that I really didn't rate. I've seen female writers write fantastic male characters and some male characters that I really didn't rate. I've also seen male writers write some fantastic male characters and some male characters I didn't rate. I've seen female writers write fantastic female characters and some female characters I didn't really rate. Is a male writing a male character going to have some advantages over a female writing a male character (or vice-versa) on account of having more insight into the "male condition"? Possibly yes... but let's remember, our insights are very, very, very personal. If the character in question is basically an author insert then that insight might be accurate and an advantage. But if your tale is set in a medieval fantasy world? One set in space 3,000 years into the future? A post-apocalyptic wasteland? Or a supernatural world where the male character is a vampire? With so much distance between characters in those universes and my lived experience I think it's quite difficult to say that anyone has any insight into what it would be like to be a man/woman in that world on account of being a man/woman in real life.
To pick up the point about why it's more noticeable with males writing females then the other way around, especially when it relates to the sexualisation or stereotyping of a character, I imagine the current media environment has something to do with that. Over recent years there's been a lot written about male writers handling female characters badly and it's been presented as an on-going narrative. In contrast there's relatively little made of female writers creating bad male characters and when they do it's generally seen as a one-off; Christian Grey wasn't seen as part of a wider pattern on female writers creating dominating male romantic leads with dubious views on consent (a pattern that is very clearly there) but simply as a badly written character.
On the "how much of you" is in a character discussion:
Setting aside the aforementioned author inserts (i.e. where the author writes themselves as a character) and the more Mary/Gary Stu take on that (where the author writes an idealized version of the person they imagine they'd want to be as a character), both of which are clearly the author writing some version of themselves, it's an interesting discussion point.
The simple answer would be to say that there's of course some aspect of the author in each character for the simple reason that the author is writing the character; however hard we try we can't separate ourselves from the characters we write entirely because we're the ones writing them. Moreover when we create a character we tend to do so to take part in a story we wish to tell... as we wish to tell that story some of that will leak through to the character. But that strikes me as too simplistic a way to look at it. Do some of the characters I write inherit aspects of me in them... be it my background, part of my personality etc etc? Certainly. But other characters aren't based on me, they're based on other people, be it ones I know from my personal life or public/famous figures. Again as I'm the one writing the character some part of me, however small, will shine through when doing so but the core of the character isn't me, an aspect of my personality or an element of my background. When I think about what a character would do/say/think etc I try to avoid looking at it as "what would I do/say/think etc?" or even "what would I do/say/think if I was a *insert character background and circumstances here*" but instead as "what would they do/say/think"; I keep them separated in my mind.
Finally, I'd also note that a lot of what I say above is basically pointless. Let me echo Cycle's words; if you're on E and you're following the rules and writing characters that both you and your writing partner(s) enjoy writing with then nothing else really matters. None of us are being paid to be here... writing here is for pleasure and if you're enjoying it then you're doing exactly what you're meant to.It doesn't matter if someone else thinks your character is badly written, a stereotype or a caricature... as long as you're enjoying writing the character and your partner(s) are enjoying writing with them then that's a good character.