I know she planned on sleeping with Istredd, but why do it like this if she wanted things to remain as they were? Why not keep doing what she did before, get with one while the other was out of the picture?
Again, as a farewell shag it made some sort of twisted sense: screw him, leave him and move on. And if Geralt found out, she could have just said that it was one last roll in the hay and he would have probably been upset but understood, since he loved her and put up with everything she did. But if she wanted to keep them both, why jeopardize the arrangement by risking a confrontation?
Her plan was terrible either way, but it just seems to me that this interpretation is even less logical.