As some of my FGR contacts have done today, I thought I would try The Seven Deadly Sins this week for no good reason. I'm mostly a pretty modest girl, so I did Lust first because it would be the most difficult for me.
Lust (or lechery) is usually thought of as excessive thoughts or desires of a sexual nature. Giving in to lusts can lead to sexual or sociological compulsions and/or transgressions including (but obviously not limited to) sexual addiction, fornication, adultery, bestiality, rape, perversion, and incest. Dante's criterion was "excessive love of others," which therefore rendered love and devotion to God as secondary. In "Purgatorio", the penitent walks within flames to purge himself of lustful/sexual thoughts and feelings.
It's very bad, so don't do it.