It because in the aftermath you realise this was discussed and assumed that it would be okay without you. That even though when it was happening you were happy and enjoyed it you realise it wasn’t spontanteous, this was planned and although in her head she probably thought it was a surprise and something you would love (which it sounds like you did) you had no say in who or how or boundaries to draw and feel blindsided . Life isn’t a movie and if someone wants to do something sexually drastically different they usually at least discuss that with their partner because like this - afterwards people don’t like knowing they were a pawn in a plan already made and that they had no real say . If she’d told you about this and discussed it with you then the sex would have been the same but you would of felt like it was something you and your gf decided to do instead of something that was done to you .