That was actually the first thing I tried. I made an NPC with depravity set to zero, and who was as much of a tsundere as I could make to try to avoid the scene. It doesn't list anything but a blank for her current corruption, but as soon as her attachment was over 1000 she would blow a random guy on every text. The first time I saw this scene was a high vulgarity character, so I assumed the same thing that you suggested.
The relationship over 100 and they won't do it thing isn't correct. It seems to actually activate the scene when relationship is high enough. My recent test character has an attachment of 1250 and she does it every single time I text her my location.
In any case, if this isn't intended as cuck fetish content, there should at least be an option to do something about it other than watch like a massive cuck or tell them not to do it in public. Even better would be an option to just disable scenes like this entirely.