Why is lust considered as being unskillful?
Imho, unskillful does not mean the same as morally wrong.

If I say something to someone who wasn't really open to hear what I had to say, doesn't make me saying this or that morally wrong. It is, however, unskillful, since the person didn't really pay any attention.

Why is it unskillful?
Because it leads to the habituation of clinging and attachment. If you want to strive for enlightenment, sex and pornography will hinder you on the path. Because of the increase of the addiction, attachment.

If you don't want to develop in a spiritual way, then it's not a question at all.

Skillful, unskillful has more to do with whether it helps you or others, whether it's helpful or a hindrance.

That's what comes to mind.
Hope it helps.