Many Christians believe that lust is just as sinful as actually doing the deed. It’s based on one saying of Jesus where he says if you look upon another woman with lust, you have already committed adultery in your heart.
This interpretation is foundational to a lot of Christian sexual thought and explains why they have failed to have a healthy relationship with sex.
I think one of the main issues with the interpretation is the meaning of lust.
Is it attraction? Is it masturbation with a woman in mind? Is it flirting?
In the time these things were written, women were widely viewed as property and desire was not a huge part of marriage. Who knows what specific sort of cultural thing he might have been referring to?
Personally I think lust is the debasement of a person for your own enjoyment. People consensually engaging in sexual exhibition and other feelings of attraction or sexual fantasy are probably not what Jesus had in mind and aren’t really harmful to healthy adults.