Well, actually, that is another one of those myths about guys that seem to be so common. If a guy hasn't had sex for a while, say a month or so, his desire for, and ability at, sex starts to drop rather than increase. If you think about it for a moment, if it wasn't like that, monks, soldiers on long deployment, sailors, hermits would all go completely insane with desire rather than insanely bored.

How long have you been married? Long enough to become a habit? That is usually about 5 years, sometimes longer, sometimes less. It happens to everyone, guys and gals alike. Slip into the daily routine, and before you know it the sex has pretty much dried up. Not from lack of love, but lack of time and stimulation. One of the things they never tell you about marriage is it is hard work to keep the spark going once making a living and being responsible get in the way. Well worth it though!

Luckily for you, the answer is in your own hands (no, not that way!). You flirt with him. Get romantic. Get sexy. Take a vacation away, if you can afford it! Try giving him a bath and a massage. Drive somewhere private and make out in the car. A candlelight meal for two, with you for desert.
Shake both of you out of your comfortable rut and back to giddy teenagers again. Once he gets back in the saddle, as it were, things will soon be much more satisfactory, and he'll start doing his part. I'd give him exactly the same advice if he was the one who asked, so for any guys reading, this is a universal answer.

You have to let the party animal out for a stroll once in a while, or it sickens and dies.