I'm a little torn on how I feel on this issue. On the one hand I get exactly how you feel. If men/women do the same job, then treat them exactly the same. But, on the other hand, as women we do have to keep covered up. I'm not sure why you are opposed to two extra capped sleeves? I know it's hot, but I know that those sleeves teeny sleeves will give you extra protection from chaffing in your armpit area and coverage from peeping eyes.
It's so easy to claim sexual harassment these days, maybe management is trying to "protect" itself? It also could be that some male coworker complained and he's being protected? Sometimes what "we" view as acceptable in the mirror in the morning, is not acceptable to others. It's no doubt in today's work world that men & women have different standards of dress code. We share that work place with others and have to make it livable.
Again, I see your point about wanting to be treating equally, but I also don't see how wearing two extra small sleeves is going to overheat you. I don't know, just my two cents...
