Why is lust considered as being unskillful? Sexual lust and pornography in Buddhism is considered as being unskillful, but for me it is not clear why.

If all parties involved consented to taking part in sexual acts, I don't see why they are morally wrong.

As a living animal we can give and be kind to others, but we also have our own needs which can be called healthy self-interest that we had better fulfill.

Why is it wrong to be primarily interested in a person on a sexual level? If we openly discuss our needs and theirs and are honest with them, I see no reason why it is bad.

Why is pornography wrong? In many cultures sex and/or pornography is looked down upon (or at least only acceptable once certain conditions are met, i.e. marriage) but the people there were in all probability consenting and they always have the chance to quit their job, too. Drug abuse or depression in pornography cannot necessarily be a direct correlation, but primarily a result of I) lack of coping skills and II) underlying irrational beliefs ABOUT STRESSORS and III) an unhelpful environment.
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.