Can you really find your soulmate after you're married?

Strange question I know but I just saw a movie (a pretty bad one mind you) that is based on that very question. And though I didnt like the movie it does make you think.

In the movie two couples who have been married a few years start having problems. One couple because the wife is more successful than the husband and is not able to devote any time to him. The second couple I couldnt quite understand. The husband is any girl's dream partner - sweet, funny, romantic, successful and completely in love with her. Yet his wife is not happy because she is looking for something more (what this something is is never explained in the movie..but since I'm not reviewing the movie here I will overlook that). Anyway so the husband from couple 1 and the wife from couple 2 meet a few times coincidentally and become friends in an attempt to help each other salvage their marriages. In the midst of this they discover that they have infact falling in love with each other.

Apparently the premise of the movie is that you shouldnt marry for compromise, and I cant disagree with this opinion. However, is it really possible to meet someone who is your soulmate once you are married. Don't you just stop looking. I know I am being naive and that many people have affairs outside their marriage. But I've never thought of these affairs as true love or finding your soulmate. I just find the idea so hard to grasp. To fall so in love with someone else when you have been in a happy (albeit with a few issues) marriage for so long - the idea doesnt make any sense.

Firstly I have never believed in the concept of soulmates. But to those who do, dont they believe it is in your destiny to meet your soulmate. Why would destiny play such a cruel joke then where other hearts are broken?

I suppose I am over analyzing what is of course just a movie, but still, I do wonder if it is possible. And all I can say is I hope and pray that something like that never happens to me because I cant think of a scarier situation. And if what happens in the movie could happen in real life, I do wonder how you know that someone is your soulmate before you marry them. How do u know he is the one? You can never be sure right? What a scary thought