Hi - i have a problem with my wife regarding our sexual relationship.?

i met my wife in 1995 and had a wonderful relationship up until we were married on 2000. my sexual relationship with her has degraded over time. During the first 5 years - we had no children that would get in the way of our romance. each day that she came home - she would just plop herself on the sofa and let the day just waste away. she preferred to have sex once a week and eluded to that this was normal. i was pretty firm with her and explained that if that was the case to forget the whole thing and that we should go our separate ways. i don't want to leave her, but i do want her to become more sexual i can not change a person but what do i do? i have 2 children and receive the same amount of sex as i was before the children were born. sot he question of children is not the issue. just last week, she rolled over and said do you want to do it because i know you need to sex; is there no interest; poor sex drive, any suggestions on what i can do?