I know zilch about Chinese fandom, but afaik in Japanese BL fandom, the seme/uke dichotomy is more about who fulfills whose emotional need*. So whether you ship A/B or B/A (or think they switch) is directly informed by how you interpret/headcanon who A and B are as characters.
From the hundreds** of Japanese BL manga I read, the focus is on the uke's emotional fulfillment. (Not counting the BL manga that's downright non-con.)
So...using those lens, I would headcanon Shen Wei as the seme because Shen Wei is arguably the one fulfilling Yunlan's emotional needs, at least in the beginning.
The "wife" thing is probably because Shen Wei's the one showing his love to Yunlan by performing all these domestic things (I die when Shen Wei lovingly folded ZYL's clothes. Dead.) I've only started reading the (translated) novel, so I'm not sure.
* In addition to reasons like aesthetics (who's taller, who's prettier) or personal preference (e.g. younger seme is my jam) ** a gross underestimate
BL
From the hundreds** of Japanese BL manga I read, the focus is on the uke's emotional fulfillment. (Not counting the BL manga that's downright non-con.)
So...using those lens, I would headcanon Shen Wei as the seme because Shen Wei is arguably the one fulfilling Yunlan's emotional needs, at least in the beginning.
The "wife" thing is probably because Shen Wei's the one showing his love to Yunlan by performing all these domestic things (I die when Shen Wei lovingly folded ZYL's clothes. Dead.) I've only started reading the (translated) novel, so I'm not sure.
* In addition to reasons like aesthetics (who's taller, who's prettier) or personal preference (e.g. younger seme is my jam)
** a gross underestimate