You keep confusing lust with a desire to marry. They are not the same thing.
And it is a sin to lust after others, period, whether they are married or not. Jesus said so.
To the pure, all things are pure. But to the defiled nothing is pure because their mind and conscience is defiled.
Funny how God made things, that the opposite sexes are attracted to each other. I wonder if as many people would consider marriage if there was no desire originally intended within it. It would probably be as attractive as a normal man wanting to marry a man.
1 Corinthians 7:8
I say therefore to the unmarried and widows, It is good for them if they abide even as I. 9But if they cannot contain, let them marry: for it is better to marry than to burn.
Paul's suggestion is that it is good for them to marry if they burn within themselves.