I read an article a long time ago about a growing phenomenon just like what is happening to you. Can't provide a link because it was a long time ago and I don't remember much in the way of details, but whatever experts were being cited said that this is caused by the fact that watching porn and masturbating is simply easier than seducing your significant other. With the latter you need to put some kind of time and effort into the seduction, whereas with porn and masturbating, you just fire up the video, whip it out, and start spanking it. No matter how routine your relationship has become, starting sex can't possibly be as fast and easy as that. According to the article, the phenomenon affects both men and women. There are apparently also women who are more interested in spanking it to porn than sleeping with their special someone.
There is also more recent research that may shed light on what is going on.
Book:
Come as You Are: The Surprising New Science that Will Transform Your Sex Life - Kindle edition by Emily Nagoski. Health, Fitness & Dieting Kindle eBooks @ Amazon.com.
Web comic covering specific issue:
www.ohjoysextoy.com: Science of Desire by Dr. Emily Nagoski & R.Stevens
Long story short: there's a list of turn ons and turn offs that you are not necessarily aware of that affect your sexual desires. When the turn offs exceed the turn ons, you don't want to have sex. When the turn ons exceed the turn offs, you want to have sex. The book is mostly for women and about female sexuality, but it looks like this particular research is applicable to both men and women.
So if you or your partner's sexual desires seem unnaturally low for some reason, check and see if anything on that list is affecting things.
Many cultural attitudes/assumptions (e.g. society telling women that the only thing that matters about them is their looks and that they aren't pretty enough) affect things on the "off" list for women (body image, sense of self worth which can affect depression/anxiety, etc.), which might explain the "Not tonight, honey, I have a headache" phenomenon.