I think it's pretty clear now that a romantic connection is part of the requirements, and being honest about it is what brings forth the petal on their lower body. The idea that romance is forbidden is likely a misunderstanding stemming from what happened with the previous generation. The mistake was that the original candidate decided to leave with the previous sage, not that they fell in love. This just strengthens my belief that the ending will involve Itsuki deciding to stay in the Flower Kingdom, or whatever its called. I keep forgetting to memorize it.
Now what would be cool is a pregnancy arc. That would be a nice can-o-worms to explore.