Hi, luring for a long time and stumbled across this post. For me, in most my relationships, I was the one not getting enough sex. Most of all the last relationship. Come to find out "he wasn't happy" for four years and had been cheating on me. So he was withholding sex as a manipulation. I know, sounds like a "woman thing to do" (sorry ladies I'm a tomboy)...well it's not. He was a Borderline Personality.
I also find most men to have this whole Madonna/whore complex as well. When you're dating them, the sex is insane for about a year and it all falls apart because they want you to be the Madonna/good wife. They see a healthy (or in my case I suppose...) over zealous sex life for a woman to be intimidating. Like she's going to go out & cheat on them. Then the men get insecure and cut you off.
As to your wife...(sorry for my rambling)...communicate with her. Talk her head off. Best way to make love to a woman is THROUGH HER HEAD. You make her feel great about her body and her looks (and everything else) and she might feel inclined. She might feel insecure about her body after the kids, etc...Did you really try to get inside her head or did you just try to get into her pants? Try the head first. Good Luck. May you stay happy ;)