I don't know. I was with soemone for over a decade and the sex was always hot.
Re: Third party. Well, every girl friend I knew who did it always regretted it. The guys I know said it was either not everything cracked up to be OR it ruined their primary relationship because she couldn't get the iamge of him with another woman out of her mind, or he couldn't get the image of her lusting over another man out of his mind. From guys I hear you should never do this with someone you care about--its better if its all casual.
There really is no need to resort to a third party--there's role playing, "special" outfits, toys, nights out of town, games, new sexual positions,music, movies etc. They haven't tried any of these avenues yet.
And then there's just biology. Sex ONCE in almost two years. Come on!