I think that if the theater is that crowded, then you should have the expectation that you're going to have to sit next to someone. IF it's that crowded. Now if there are open seats somewhere else, different story.
But if you are preventing another couple from sitting together just because you refuse to move ONE seat, and that's the only possibity for two together? Then yes, I think it's incredibly rude to not move down.
Why? Because if it were me (and this seldom happens, as we are always early as well), then I would hope that someone would be nice enough to move for us.
I understand getting there and staking your claim. I do it too. But if it's so crowded that there are no other seats together and you can help people out, why wouldn't you want to do that?
Thank you everyone for your comments. This seems to be sorta the consensus.
I think the things that bug me most about it is this:
1. My husband and I have sometimes arrived at movies and been very close to the start time, but not actually late. If the theater is crowded and there are not 2 seats together, we just sit in 2 separate seats, I would not ever ask someone else to move. I'm not sure why it's so important for people to sit together in a movie. You're not chatting during the movie (or not SUPPOSED to be), you're not making out (ditto), you're both sitting there staring straight ahead watching the movie. What difference does it make if you sit next to your beloved or no? I know it's fun to hold hands, etc., but it's not the end of the world to sit apart. We have also sat apart on subways and busses, etc., rather than ask people to move to accommodate us. To me, it would be rude to ask people to move if they are there first, so I don't do it myself, and so I guess I expect other people to play by the same rules.
2. On some level I feel like the late people are taking advantage of us early-comers. (BTW, this incident happened after the official "start time" of the movie, but of course we were still in preview-mode so the movie had not actually begun.) I feel like DH and I got there in plenty of time and sat sat sat sat sat sat waiting patiently, and then other people just assume they can linger at the store or the restaurant or their own sofa until the last minute, and then just WALTZ right in and tell other people to move and the rest of the world has to accommodate them. So I think it seems unfair to me.
This was the first matinee on New Year's Day for a popular movie that had been out for several weeks -we do always try to hit the first matinee to avoid the crowds and assumed most of the world would be home nursing their hangovers. NOT. Anyhow, we saw another movie over the weekend (The Blind Side, LOVED IT HIGHLY RECOMMEND) and cunningly sat RIGHT IN THE MIDDLE of a row so we would not have to face this again ;-)