How important is sex?

My wife and I have two kids together. Ever since the second was born our sex life went down the drain. We went from 4-5 times a week to once or twice every week to two. If we don’t pre plan it it just doesn’t happen. I like being spontaneous but now it’s impossible with two kids under two and she tells me I make our relationship about sex. When all I want is some alone time after we take care of our kids and they go to bed. Am I asking for too much ?