I'm also never sure what Shen Wei's reasons for avoiding PDA are, and just now I thought if he's simply worried that he can't control himself in public? He's always far too worried about that, so maybe that's the actual reason, or at least a part of it.

That would make sense!

I tend to read it just as Shen Wei being an inherently private person who doesn't want to air his laundry (dirty or otherwise) in public, and is falling back on conventions to justify that. Plus he's harboured this secret love for an unimaginably long time -- it must be so hard to suddenly have it in public view and treated almost jokingly (though ZYL is very sincere underneath).

But then, I'm not very good at taking novel!Shen Wei's nature into account. Your reading makes more sense in some ways!