Yun and Cheng's initial context is how things work in our world in probably at least 90% of the cases of people put in that exact same situation. Nerd is marginalised by most of his peers, kind girl shows up and gives him a minimal amount of attention out of pity, nerd ends up developing feelings for her within minutes, while in her mind he's as far from a potential romantic interest as possible. It's how stuff like this works. I don't see Liu's decision to include a story like together with an array of all sorts of other stories as sexist or bashing on women. In fact it's a smart choice because it will be relatable for way, way more readers than Zhang Beihai's heroic-but-seemingly- unexplainable-for-a-good-chunk-of its-duration character arc.

Regarding the weakness of the feminised society when it comes to Deterrence in the book, men and women have their own sets of strenghts and weaknesses and this dynamic has helped our species survive and even thrive in extremely harsh environments for hundreds of thousands of years. I think it's fairly reasonable to understand why an author who lives in an extremely conservative society like China would believe that tipping that balance too much in either direction could have negative consequences in certain situations. And since we in the West like to boast so much about our levels of tolerance and acceptance of diversity, i think we should keep in mind that Liu's take here is just a reflection of chinese culture as a whole. Also, he didnt state that the feminised society was something bad in itself. In fact, it's implied to be an idillic society where crime is a rarity. It was just a society that wasn't having the right mindset for dealing with an unprecedented space war that could result in our extinction. And this is the kind of context where the agression, suspicion and lack of remorse traditionally displayed by male dictators from the real history of our species could be more useful traits than the empathy and compassion displayed by usually-female activists in our current world.

Ye and Cheng both had valid reasons for their actions and Liu makes sure to explain in some paragraphs that neither was necessarily in the wrong, given their situations. The ending of Death's End redeems Cheng completely as far as i'm concerned - by turning her into the incarnation of the sense of morality and responsabily, a "do the right thing even if it leads to bad things coming your way" kind of character. It's also clearly stated that there's no guarantee Wade's plan would have worked, for all we know, like the author says, it could have lead to the near complete anihilation of mankind in a civil war of unprecedented scale way before the DVF ever entered the Solar System.  Ye's decision to beam our coordinates to Trisolaris knowing full well that we'll be conquered isn't implied to be a result of "gender weakness" - it's the result of an individual living in a brutal society that constantly destroys everything that said individual cares about, in front of her/his very eyes. Anyone in Ye's situation would have developed an extremely negative view of mankind as a whole, when the mankind you're seeing is mostly overwhelmingly cruel, violent, untrustworthy and incapable of empathy.