Sometimes people who have all the necessities in life to sustain them are not content with what they have. Some people already have more than they need and want more. Lusting. It's more about the attitude of one's heart towards what God has given to you.
And the mixt multitude that was among them fell a lusting: and the children of Israel also wept again, and said, Who shall give us flesh to eat?
(
Numbers 11:4)
As God was bringing the children of Israel out of Egypt, the way of life was disrupted and the normal cycles of food production was temporarily disrupted likewise. God fed them with manna. It most likely contained every nutrient the body ever needs and had a fresh oil taste. Good grief! They had food that all they had to do was to go out in the morning, pick it up and eat it any way they wanted to fix it. The children of Israel knew that they were in transit to a land of "milk and honey", yet they complained about what they did not have. From what Moses said about them, I can gather he thought of them as like little whiney babies crying out a tantrum.
So if you believe God in His wisdom was giving you everything you need to sustain life and if you married the spouse you believe God gave to you and you started looking at another with lust in your heart after her/him, then yes you commit sin. If you are complaining about what God has given to you as a blessing, it stems from not giving God the thanks that He so rightly deserves.
Anytime God blesses a person and they complain about it, wanting more or something else, then that is lust.