From what I can tell you need to have a certain amount of lust on your own character for them to pop up (mine started to be dealt around 24-48 lust)
If they still aren't, I'd try to add more cards to your deck during battle, either by using the reshuffle button 🔄 or an inspiring brew
Lust card strategy: the best way to utilize these lust cards strategically is first getting the enemy to a certain amount of lust where they start showing the horny trait (you can either use these cards that you're referring to or you can use other cards that do damage and increase lust)- after they show that trait hit them with denial (if you have it, if not you can either learn it when leveling up or learn it when using certain weapons) and it should do pretty decent damage
It's important to note that if you or the enemy maxes out on lust then you/the opponent will lose all energy and mind by the end of the lust max cooldown