If you must, one of you should have short hair, wear jeans and hiking boots, and walk firmly. People will assume you're a man. Don't look like you're enjoying each others' company so much. Stifle your laughter. Don't wear the same clothes as your lover, even though you bought them together.
People tend to put things together a little easier.
Lose the backpack, jeans, and swagger. Don't wear political t-shirts or buttons. They're a give away. If you wear shorts, either put on knee socks that stay up, or shave your legs and pretend it feels better that way. Wear something other than Birkenstocks, Doc Martens, and running shoes.