My guess? Timing matters. The swan conversation come right after SH decides she wants a relationship. The cheating happens some time later. I've only seen it on youtube but it possible that Tav helped her rescue her parents and got the Viconia off her trail.
The relation is stronger at the cheating point. The swan line is a bit cringe but it seems an insightful exchange - seeing Tav cheat on her with a devil is likely to push SH back towards Shar and so memories of life the cloister return. If Tav is smart she will try and heal the wound the cheating inflicted.
SH is being generous but she's feeling hurt. Tav should have asked and should have chosen a different partner.
edit: obvs she flirts with and sleeps with Halsin. That's what we're discussing, right?
