Guests wearing shorts?

What about listing attire as business casual?

I'd probably say something like "We know it's summer and the weather can be potentially pretty hot however we are still hoping to see our guests a little dressed up ". I'd make it suggestive that shorts is not what you'd love to see but not enforce what someone should wear (if that makes sense). I think it's delicate enough and hopefully they should get it? I know there are such mixed opinions on telling someone what to wear but if they are asking, I see no harm in answering them.

This reminds me of when a guest asked if they could bring their child to our wedding and instead of saying "no kids" I said, "we are looking to give the parents a fun night out to enjoy!" and they got the message, thankfully lol. Good luck!