So, if you're a lust-feeder, simply touching someone who's aroused might be a taste (see Thomas' time taking "tiny bites" from women in his hair salon), to feed properly from a single target, you have to have sex with them.
In Backup,
Thomas kills a Stygian by kissing her on the neck and draining her life force.
That's not quite how it went.
While it doesn't go into graphic detail, it's not as simple as kissing her neck. "I bore the Stygian to the floor and fed her to my demon" reads very much like a "fade to black" before a sex scene.