It's not as hard as they think, because it's not about them.

On a forum I frequent, a poster asked whether her husband's reactions to students clothing in a hot climate where both men and women bared a lot of skin were appropriate. In this, a remark came up that I've heard before many times:

"It makes me feel SO sad for men in today's society. They are expected to figure out what message a woman's attire is sending, and react (or not) appropriately, and if they don't, they're sexist pigs."

I bring it here because this is not unique to this conversation at all.

In reality, the expectation for men is a lot simpler than a lot of people, including this poster, realize.
1) Is the woman in question the man's significant other?
2) Is the man in a common "pick up" location, like a bar or club or whatever is common in their area?

If one of these is yes, then the man may need to decode a message.
If the answer to both of these is no, then the answer is easy. The woman isn't sending the man a message at all! She's just wearing clothes. There is no message for any strange man to figure out. Just leave her the hell alone. What she's wearing is no one's damn business but her own.

There's no real complexity, just a societal assumption that everything women do, especially with their own bodies, is for the benefit of men. And yes, that assumption is sexist.