It's like "great minds think alike", but a girlier version. You say it when someone else was thinking about the same thing or wearing something similar as you were.
So you and someone else must be very sexy and fabulous because both of you thought of the same thing or agree on something fabulous.
Girl1: "Omigod you're wearing an animal print dress and a tiara for your birthday party? I did that too for my birthday! Sexy minds think alike!"
Girl2: "Sexy minds do think alike!"