I've learned that rule the hard way.....NEVER promise it! Promise that you'll try, but don't promise the actual deed! I have a really hard time getting in the mood with this pregnancy, and there are some days where I just can't fake it through. I'm also more likely to get in the mood, if I know it's not expected of me, if that makes any sense.
Poor OH. When I was pregnant with DS, I wanted it ALL. THE. TIME. I think he feels a bit cheated this time.