They say if she's not doing you, she's doing your best friend. Short of that she's totally pissed at you for something menial and is using sex as a weapon until you grovel at her feet.
Some women also lose sexual desire as they mature but I'm sure this is rare, but my guess alot more frequent than with men
Also I read this somewhere and I tried it, it works:
For one week do all kinds of cool stuff for your lady, the things she usually asks you to do. But this time just surprise her with these things she wants, all the while remind her how smart and unique she is. Not sexy or she'll smell something funny. Flowers, little attentions, etc. After about a week of this chose your setting and you will get your killler 'aerobics'. Good luck!