A timely topic. I turned up for dinner in what I thought were acceptable (tailored, not sports) shorts recently on Constellation and the hostess discretely and with great sensitivity asked me to change as shorts weren't allowed in the MDR. I was more than happy to do it and assumed simply that the people I'd seen on previous nights in shorts were perhaps victims of lost luggage or similar. I didn't ask, but assumed that there must be some special dispensation.
However, through the cruise I noticed people (men and women) in the MDR in shorts every night. Some in sports shorts (and one lady, on evening chic night, in a bathing suit beneath a one piece cover up/Sarong) Evening I concede, but Chic it wasn't. Maybe they'd been challenged and declined to change, or more likely the hostess chose discretion over valour, but the end result is the same.
Generally, I don't worry too much about what other people wear, but I do question the inconsistent application of a rule or guideline. Allow shorts or don't, but be consistent. That's surely the only sustainable policy?