For years, my great-grandmother introduced her boyfriend as their boarder or family friend. He lived in the spare room in my great-grandparents' little house, and I remember him as sort of distant, although he did used to play with the kids. It was only years after they all had passed away that one of my grandmother's cousins told me the guy was her boyfriend, and that the whole family knew (I was way too young to notice anything weird). I got the sense from my relative that my great-grandparents just acted like it was the most normal thing in the world. But to friends and neighbors, he was just a boarder.
You just introduce people to your friend, act like it is any friend (that means wiping the guilty grin off your face, LOL), and be careful with any public displays of affection.