is little sex normal?

I've been happily married for 2.5 years and sex is really good except it happens twice a month, we have no children... is that normal? I'm 27, he is 33. I could have sex more often and be happier but either something happens, work, we're tired or busy and if I initiate he doesn't really like it, I need to wait til he tells me he wants to. I don't like that but I've gotten used to it, and sex is good but I keep wondering what ever happened to our 3 times a week before we used to live together...