Is it okay for my boyfriend to watch a movie with another girl late night in his room?

Answers :-
Opinion 1:-
I have a girlfriend (the rather charming girl in my profile picture), but my flatmate at university is a girl (and my best friend).
As such a I spend quite a few evenings watching tv or films, doing work or whatever with a girl who is not my girlfriend.

I had a long chat with my girlfriend before I signed up to this - because I love her and didn't want to hurt her. She was very supportive of the idea because she knew how good friends we are. There is nothing even slightly sexual between me and my roommate - she's essentially my sister!

However I do still worry lots - it's been quite hard, not because I am tempted, but because I worry that she will think I have been tempted.
What I am getting at is that it doesn't necessarily mean that anything is going on - guys and girls can be really close friends without any sexual hijinks, but the key is communication.
Talk to him. Tell him you're worried.
I wouldn't jump to conclusions - because I've been in the same situation and it's been entirely innocent. I hope it's the same in your case.
Remember: communication! :)
A few of the comments have stated that they simply do not believe that there is nothing between me and my roommate - so I've decided to add a bit more to my answer.
Let's get one thing out in the open:
I do not want to sleep with my roommate.
She's a really pretty girl, but even if we were both single I wouldn't want to - I just don't see her in that way.
My girlfriend accepts that - and after that has been accepted, then there's nothing more to worry about than if I was sharing with a guy (there's an ongoing joke that I'm quite camp and that she'd need to worry as much if I was sharing with a guy!)
People have also attempted to infer some things about my relationship and how it has evolved since I've moved in- which is sweet of them, but given they don't seem to be able to grasp the core concept of my relationship with my roommate (see: no desire to sleep with), I think I'll leave the judging to my girlfriend! However I will say that it has brought us closer, given that the fact that I talk to her about my worries (that I think my girlfriend might think I like my roommate) has made me open up to her in a way guys don't really seem to do very often - hence the added closeness.
My message however, remains the same - if a couple is open with each other, talk about everything and is as honest as possible - and you both respect the boundaries you agree on, then don't let other people tell you what is and isn't ok about your relationship. If you're both happy - then go you! If not - talk to each other. If there doesn't seem to be a way to resolve the issues - then maybe you need to have a think about the relationship as a whole.
I wish you all the happiness in the future,
Opinion 2:-
Instead of answering your question, I'm going to answer the question you should have asked: "Is it okay to be upset at the idea of my boyfriend watching a movie with another girl"?
That's the far more relevant question.  As the other answers point out, there's nothing innately wrong with them watching a movie if they have no romantic intentions or attraction, but it's a situation fraught with danger and implications.
Moreover, even if nothing happens, it sounds like you don't like the idea of them doing this.  And that's something your boyfriend should anticipate, and which he should respect.  You don't own your boyfriend, and you can't tell him what to do, but you have every right to feel the way you do.  It's entirely reasonable for you not to want him to spend time with a girl under those circumstances, and if he's any kind of a boyfriend, he'll respect that.
This is the kind of situation where what's acceptable is a question of what the two of you are comfortable with.  If you're not comfortable with it, he should not do it.
Opinion 3:-
It really depends on the relationship, what kind of person your boyfriend is, and what is the relationship between him and the other girl.
I wouldn't mind letting my boyfriend and his best girl friend (if he did have one) to live together, not just be in a room together. Because I know I totally would love to share a place with my bestfriend, who is a guy. And I am 100% sure that our relationship is beyond the friendship level, but it is going the totally opposite direction of a romantic relationship. So there is nothing to worry about.
However, if my boyfriend knows some girl for about a week, or a month, and he wants to do activities, which we usually do together, with her, I would not be fine with that. It doesn't necessarily mean that I don't trust him, it only means I am human and I am allowed to accept what I feel. If something makes me uncomfortable, then I am uncomfortable. But then I would let him know and see what we can do about it, instead of getting mad and ruining everything.
Every relationship is different from the other, and communication is always the most important skill.
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