Yeah Fuuko is obviously not making a booty call here and that's not why Yuni's going to meet up with her. Comments acting like it's simply "cheating time again" or something makes me wonder if they're reading the text or just looking at the (very well-drawn) pictures.>
I don't think anyone is saying that Fuuko called her for that purpose. Fuuko even acknowledged that she is scum by wishing the baby turns out differently. She might actually learn and grow.
Yet Yuni, well, she is still Yuni. She did without doubt run away to do whatever Fuuko wants to do. While Nanase was still trying to find her. Total asshole move, and I would expect nothing else from her character.
Yuni and Nanase should break up, but after Nanase gave her another chance, she could at least show the bare minimum of respect for a person who cares for her.