Kids + Marriage can be death on your sex life.
Also... (definately not a woman rant... I'm definatly pro-women!)
I honestly believe that many (if not most) women do not want frequent, erotic sex on a long-term basis. If you're married to one of those who don't want it, your next best bet is for the woman to recognize that most men do and to be proactive about keeping things going.
I spent 16 years married in a heady, intellectual-relationship (we both met in engineering school and were married before starting graduate programs). I wrestled with her "no touching me before 8:30 am", her barely moving during sex, and even asking me to try to finish faster (which is the *ultimate* desire killer). Sad thing is, she still doesn't completely understand why I'm leaving. Sex was an option just like going to a movie or running to the mall -- she could take it or leave it.