It's interesting that you point out that they arrived late... because even if they arrived before the movie started, you & your mom might already be in your seats, & then there would have been even more other seats available.
Here's my take on it, for what it's worth:
At a movie theater, many people like to sit the same distance from the screen. What is too close for some is just right for others. My favorite spot is front row of the elevated section, center screen.
So maybe there were other seats, but they were further back/forward from this couple's ideal. But really, is it a weakness that you would move over for them? It sounds like you both sat in seats that only allowed 1 person to sit next to you... what was the purpose of that? Were you trying to bully other movie-goers by claiming space beyond your actual seats?
I'm not actually razzing on you, just posting philosophically... I think it's interesting that there are so many aspects of this situation. Certainly the view from you/your mom... but also from the other couple, & then from the other people in the theater.
I think that if the couple is not one you know, they did not intentionally run late just to make you move, & they asked kindly, the kind thing to do would be to move over for them. I personally do not see that as a sign that you are weak & a push-over. Instead, it's a living example of being part of a polite society.
Circumstances where I would not move over - they are selected seats @ the theater (some allow you to "pick" your seats assigned ahead of time), these are people you know to be mean & badger you to accommodate them all the time, the couple could have sat in two seats to the side of you but wanted to be closer to the middle & thus asked, the couple wanted an empty seat between them & you (for coats?), etc.
I think an overall great mindset is "do unto others as you would have done unto you", & many times the reason we "stick" & do no do so is because others have not done for us... so we take it out on the next available situation.
T.