Sex itself is not sinful-- it is a gift from God given for us to use in marriage–using it outside this intended purpose is mortal sin. Looking at a naked body in and of itself is not sinful either. The sin comes from within you–from lusting. From using that person for your own self-gratification whether in your thoughts, words, or actions. That is the point you stop treating that person with the dignity they deserve as child of God–as your brother or sister in Christ and begin treating them as an object for your gratification (this is the mortal sin).
In this day and age we need to guard ourselves from the near occasion of sin of lust, especially those who are weak against the temptation (FYI, I was one of those people at one time so I have a bit of experience). Marketing people target us with the temptation of lust to get us to buy their products.
Compare a billboard today with something like a painting of Michaelangelo. In Michaelangelo’s work it is simply a naked person, painted as to be realistic and beautiful in a non-sexual way. Where many advertisements today they have more clothes but are made to draw your eyes to the breast or groin to get you to lust. Because of this you may find your eyes caught for a few seconds by a billboard, but once you realize it is happening you must stop to avoid sin.
It is not good to tempt with sin. All sin saddens God, regardless of whether or not it is mortal. St. Francis De Sales in “Introduction to the Devout Life” talks about getting rid of your attachment to sins. You don’t want to just avoid sin, but you want to be disgusted by sin. He mentions that some people although they avoid sin they still have an attachment to it and they crave it, and are jealous of those who commit it and would commit it themselves if it wasn’t a sin. For me, this was a difficult way to live with lust and I would have not been able to say no to the temptation if I stayed in that state myself. Thanks to the grace of God with enough prayer, the graces of confession, and deep thought on the church’s teaching and seeing the truth of how these people in these photos are being degraded it actually makes me a bit sad to see it happening today. I hope this helps.