Someone would have to explain to me why it would be a "bad decision" for James to do more movies which also have Anne Marie in them. I can't see anything wrong with that.
Filmakers don't cast James and Anne Marie because they are a married couple....they cast James because he is the best for the part and they cast Anne Marie because she is the best for the part. That they are married is just a coincidence. If filmakers (and most critics) can look past the fact that they are a couple, why can't so many of James' fans? I can't shake the feeling that in their heart of hearts it's because they don't want to have to see them together and be constantly reminded that James is off the market.

But shouldn't it be about what works best for the project? James and Anne Marie are wonderful actors, both in their own rights. They are an asset for any project. Just reducing them to "a couple" is a bit unfair, I feel. Granted, there have been movies in the past where real-life couples just didn't work on screen, especially if they were playing couples in the movie as well. Tom Cruise and Nicole Kidman come to mind. But there have also been wonderful examples where it worked very well, right down from Elizabeth Taylor and Richard Burton.
If Anne Marie was just an actress that happened to be in several projects with James over the years (which wouldn't be too unusual in Britain), people would applaud her for how well she fits together with James.

It's about the characters, not the actors portraying them. From Shameless I got the strong feeling that James and Anne Marie were constantly pushing each other to new heights as actors, and I found that riveting to watch. I'd definitely like to see more of that on other projects, because it can only be good for their performances.

I think people need to learn to look past the fact that they are married and just enjoy two great actors that work very well together.

But I also think I know what's making that so difficult. I think that in this age of the internet, of TMZ, Perez Hilton and Just Jared, of instant access to pictures and informantion on the web, we as fans tend to be too much invested and involved in our stars' private lives. The more your fandom becomes about the man James McAvoy instead of the actor James McAvoy, the more you loose the ability to see the characters he's playing. More and more you start seeing him playing a character. Likewise, his performances become more and more infused with all the information you have about the private person James McAvoy, like for instance that he's married to his co-star. You think of the pictures of them shopping for groceries you saw just yesterday. You think about a comment James made about Anne Marie in the latest interview you read. And more and more you're taken out of the performance. You stop seeing Robbie Turner, Nicholas Garrigan or Thomas Lefroy, and you start seeing James.

I'd guess that for all of us, our James fandom started with seeing him in a movie without knowing who he is and being instantly fascinated. We go looking for other movies he's been in and are equally blown away by his other performances. Then, and I really think that's a fan's downfall, we start looking on the web for information about him and find out from watching/reading/listening to countless interviews that apparently he is a nice guy, too. If he was a major jerk, we could just happily go back to enjoying his movies, nothing more. Instead we become infatuated with him as a person as well, not just with him as a great actor. And then we find out that he's married. In a way, it's only natural to be jealous of Anne Marie and envy her status as James' wife. There's always this feeling of "I'd be so much better for him" that we as women can't really do anything about, even if intellectually we know that it's silly. I just think it's sad when we let it come in the way of enjoying what this is all about first and foremost, which is great acting.
I think sometimes it's really important to cast away all the "star" stuff that occupies us so much and just remember that it's about the acting. I wanna see great acting from great actors in great movies/TV shows/plays/whatever. I'll be damned if I allowed the fact that so-and-so is married to so-and-so to spoil my enjoyment.

(Forgive my babbling. I've drawn in large parts from my experiences of eight years of Russell Crowe fandom.....but I find that so many things are always the same wherever you look.)