It depends to what degree it is,
Holding hands and smiling: That's so sweet.
Kissing or cuddling: Aww how cute.
Tonging and hip wiggling: Wow they must really love each other...^^;;
Groping and making out: Um...okayy then... -looks away-
As long as they don't start undressing and getting to that point I don't care that much.
Look away and move on. It doesn't effect you so why do you care if you can just walk away?