Or...Shadowheart is an adult individual who can engage in sexual jokes and banter without it meaning that she is going to act on any of it or even want to.
She is more than happy to engage in some lighthearted romantic fun at Druid's Grove. She seems well aware that she is attractive and used to responding to advances and banter with witty remarks throughout the entire story. The moment she shifts to being a bit more serious and sincere about it is when she and the main character realize that there may be some real and serious feelings involved. This is actually good story arc writing because it shows that the character is vulnerable and responds to the raised stakes of having her heart broken with shifting the way she addresses things with the main character. That she stays the same with regards to responding to other people that she does not have a romantic investment in, and doesn't change her entire person to submit to the main character's perception of how a monogamous person in a committed relationship must act and behave from now on, is indicative of a good and healthy adult relationship. As long as she does not go ahead and sneak off with any other other person behind her partner's back, even if she actually wants to do that, she is showing respect and commitment to her relationship.
There, I won't even charge you for that relationship councelling.
