Sorry, I just don't see it. 6.9 is a mighty high rating for a 33-year-old romantic comedy. Most people gave this 7 or higher; I gave it 9/10.
I suspect that more likely, the minority who didn't care for this didn't like the slightly "stagey" aspect of the adaptation of the play to the big screen, rather than the theme of infidelity. But I suspect most, like me, were OK with both aspects.
I didn't think of it as a romantic comedy at all. Take away the child delivery scene and it's a staight drama to me.
You're right: It's more romantic drama than comedy, especially because parts of it are rather sad.