QuantumT, while that type of a reaction should not be exhibited by either homosexual or heterosexual couples I do understand where they're coming from. Though don't get me wrong, you are right that you did them no wrong and deserved to enjoy your stay at the theatre.
It's hard for anyone to assume the rare case as standard. You are one of that other, smaller part of the population who can frankly and very respectfully say "Hey, I have no problems with you showing your affection for one another, and regardless of your sexual orientation I want to watch the movie without having any distractions. Thank you," which is respectable. However it's very easy for them to assume that you're just another one of those people that are absolutely disgusted with homosexual behaviour and it should be let known that they've probably already had a hard time to make it there and be with each other in the first place. They most likely receive no support from family, little from society and while heterosexual couples can rest easier showing their affection for each other at home, I can guarantee you that more homosexual couples, especially young ones, do not have the same luck.
It's sad to know that they reacted like that, and should by no means be expected from homosexual couples. And once again, though it's hard to assume the rare case, a peaceful reaction from homosexual couples, to be standard, one should consider the life of classical conditioning that constitute the lives of homosexual couples. They've suffered a lot but there is no need to take it out on others.
One day we will live in a world where that type of reaction ceases to be seen, because there will be one love, one from the soul and not solely from the body. Until then QuantumT, I trust you will continue to respect homosexual couples and I hope that if you encounter a PDA-happy couple, that it be a more understanding and respectful one than the one you encountered in that incident.