My ex and I ended up splitting because of this very issue... but mostly because my ex is VERY mono and couldn't conceive of being in a relationship where there were other relationships allowed. We were married for 13 years when we decided to split, and our divorce was final last November (after technically 15 years of marriage). We do not have children. The split was VERY amicable, and he is still my best friend, we just do not live together anymore (and in fact, he moved clear across the country).
There were other factors as well (I am also much happier now that I am no longer financially tied to him), but the main one was that we basically were in a sexless marriage for almost the entire duration of our time together. I couldn't live like that any longer.