I'm gonna guess one of two things are going on:
1. That most/all vampires in this setting have some kind of hypnosis effect so they can make someone fall in love for the first bite, and Nazuna doesn't want to use it and wants to let him fall in love naturally.
2. The trigger is actually sex while feeding, not love, and Nazuna's being romantic and doesn't want to have sex without an intimate connection.
Given how they gave him his choice of vampire, I'm leaning toward the second one.