If you are a man dressed as a man, they will send you to a male dressing room no matter the clothes you're bringing in. This is, of course, provided that there is only one dressing area with only a difference of a right of left turn deciding where the man or woman goes (as Walmart has). Now if you're in Victoria Secrets that a different story, but even then they will let you in.
I'm not a lawyer, but my guess would be that they can NOT turn you away based on the clothes you're trying on. I'm pretty sure that would be a nasty law suit. I know my wife spent some time working for Victoria Secrets and they men come in all the time. What ever they wanted to try on they could. No one ever stopped them.
Many years ago, I worked at the GAP (in Florida) and I remember a British guy came in with his family and was desprite for a pair of shorts. The male shorts were too baggy and long for him, so he tried on an purchased the largest pair of womans short shorts that we had. He wasn't like us, he just wanted shorter shorts. Point is, we didn't stop him...it was a sale and money does pay the bills after all.