I'm not sure where the line between "lust" and "acceptable desire" is, and I suspect most Christians aren't either - although marriage and procreation usually seem to be involved somewhere.
There are far more unconditionally sex-positive religions.
Most Christians fail to live up to their own theology, however the thing itself is beautiful. In Catholic terms, lust is precisely the desire to have or use the other apart from love.
In theological terms an 'evil' is always a privation -- a lack of good and in this case a lack of love. It is a reduction or removal of some vital piece.
Just as a healthy garden has many necessary parts: soil, light, water, protection, seed, etc. healthy sex should have all of the following: Love, stability (via Marriage), openness to the possibility of creating life, be unitive psychologically and spiritually, desire the good of the other above self, pleasure, and a sacred character.
Lust undoes most of that leaving only pleasure intact. Lust is about taking. Love is about giving. Appetite placed above the good of the other is disordered (ie: an improper ordering of priorities).
From a theological perspective (which may be different from the social norms that a given church teaches) I think the answer would be that it is bad if it is outside of marriage, not done in love (e.g. objectifying someone), or if it becomes more important to someone than God (which would be considered idolatry).
There is, at least in the Catholic tradition, explanations for pretty much every teaching. From the doctors of the church (the most famous being Augustine), papal encyclicals, and ecumenicla councils. It is all very philosophically and scientifically grounded. You might disagree with the reasoning or a base assumption (natural law) but there are explanations. Normally it has to do with how your action effects yourself and others. In this case objectifying someone causes you to perhaps treat them poorly and prevents you from seeing the inherent dignity in them as a person.
I’m not making a comment on this vs. violence because I think that’s a much more cultural thing in America and I haven’t looked into the Church’s stance on portrayal of violence, if it has one.
I'm not sure where the line between "lust" and "acceptable desire" is, and I suspect most Christians aren't either - although marriage and procreation usually seem to be involved somewhere.
There are far more unconditionally sex-positive religions.