Can a platoon sgt tell a female soldier that she is required to wear a bra when in civilians?

it depends a lot on the duty station that the soldier is assigned to decide on dress and appearance.
if you are stationed at an embassy in a locale where most women wear a bra then I would say "yes".
if your in a combat environment and dressed as a soldier then it is discretionary.....

It's Never a question of what you wear but always a question of is it professional (perception) to an iindividual So ask about the instructions and talk to you 1st sgt in private to get advice.

I retired in 1988 but the last ruling I saw on this topic was this. Underwear was mandatory for all soldiers while in uniform and/or while dressed in civilian attire during "on duty" status. The wearing of underwear is a matter of soldier preference when not in uniform and in an "off duty" status. Is this really what our present day soldier worries about?

So, there are “special rules” if you are in rotation in a foreign country that supersede regulations and those rules are hashed out in joint military and state agreements. I learned this the hard way during a rotation to Kuwait. They expect a certain dress code and that code is actually part of the joint military and state agreement. Your platoon sergeant may be getting word from on high to tighten the ship about things like this. Though you may not find it in Army regulations, there are things some of the host countries expect you to follow anyway.

My response to this, speaking as a former PSG, is to read up on any local policies before making an issue, otherwise a PSG or any other leader for that matter can't enforce personal preference as policy. If you're anywhere other than an ultra conservative place, like the Middle East, the only people you might offend are Americans, because we are the ones with these hangups. Ask the PSG if the males are required to wear underwear/jock straps to secure their private parts, if the answer is no, that's the end of the conversation. It sounds to me like this PSG has a personal issue and that's not enforceable policy. Pick your battles, but if it's worth it to you, then follow the chain of command/concern and use your available resources, like approved policy letters from garrison or regional commands and the IG, it's not accusing someone of harassment, nor is it disrespectful or insubordinate to ask for clarification and seek further guidance.

All good points previously stated. In my opinion, this is an opportunity for you to have a professional discussion with your chain of command and be part of setting the standard for future reference. Discussion points should also include whether this is an accepted standard in the local area.

Please remember that you represent the United States wherever you go, and your decisions, however big or small, have power to influence the general populace either for or against our nation’s presence there. It’s a stretch to believe that each of us have a hand in shaping international relations, but we do. Keep us posted on how this develops.

While it seems inappropriate to address this matter as an official act, when you are a member of the US military you should be open to making your life as a soldier easier by following that input. It costs you little to comply, and until you leave the service it might make your professional relationship more tolerable, despite how you feel about it. I would suggest not making this an issue, and that will minimize the effect of it on you. Don't get upset, or offended by this, just do your duty to the best of your ability and move on. This situation is temporary and you might find the next command you are under will not have an issue with this. If they do, I suggest that you take a different perspective and follow your leadership's direction, unless it becomes more outside of the scope of regular military duty and regulations.

I FIND IT AMAZING THE PLATOON SGT DID NOT TELL YOU HERSELF, AND USED A MALE TEAM LEADER TO PASS THE WORD!
MORE RANL AND NO GUTS???

I like that idea! Maybe your ePSzAR can see if you are ready for MPADAPCO1 training on the V158W8?

Wow! There is a problem as a Marine (1967-70) that I never ever thought about. This is the new modern Army I guess. From my perspective and this is just as a typical retired professional, I think that there are a couple of things to consider. First, I'm sure that no matter how progressive the Army is these days, that at some point they gave you the speech about being an ambassador of good will when you're overseas. Having said that, I think it would be advisable to consider the culture of the country you're in as well as some other things like weather. Going without a bra is probably not a good idea. You did say that people could tell that you were without a bra. When you're out and among the people I can't think of many cultures that would view your mode of dress in a positive light. In my opinion I can see the possibility of lots of problems. I know, not for you. I'm just giving you my opinion. One more thing. I'm sure that you realize and understand the issues and that concerns me more than anything else.