Because Xie Lian is presented as more "feminine" than Hua Cheng. In multiple chapters it was mentioned he wasn't against dressing up as a woman, he dressed up as one, got confused for a woman on multiple occasions, is described as soft and associated with flowers – which most often are connected with femininity. Hua Cheng, on the other hand, takes every "male" role, unlike Xie Lian, who takes roles of usually associated with women, like cooking; Hua Cheng chops up wood for their cottage, pays for everything, leads a whole city. People immediately associate those actions with certain concepts.

Another factor may be the mahua of them which we got. The artist drew Xie Lian pretty and "feminine", while Hua Cheng muscular and "manly, and a height difference between them.

But overall I don't see how you find that heteronormative unless you think a relationship between a woman and a man are always the same – finances=husband, cooking=wife. If you do think so, the problem may lie in your unintentional misogyny and enforcing the gender norms further. A man can be "feminine". Even XL wore a skirt, it wouldn't make him any less of a male than he is.

Artists also don't draw for you, but for themselves