When I worked in retail we used to have a couple that came in regularly, and it was obvious they were men. This was no problem to me or any of my staff. However, there was one time when they wanted to pay by credit card. I was presented with a dilemma, the card clearly said Mr. I was in a bit of a quandary, did I say something, even though I knew full well, or just leave it? I left it, the signature matched and I knew. Just a little awkward, not knowing what to do for the best, and not wanting to offend or upset anyone.