To be honest, it's the out of character that bothers me most. Up until then, Anita had gone from 'not until marriage' to 'it's OK if I'm in a commited relationship with you'. There was nothing in her character development up until that point that would make me think she was the sort of person who would have casual sex in the shower with a stranger. So, if LKH wants me to believe that it was all totally consensual, no mystical forces compelling either of them, and Anita was totally into it, then what she's basically telling me is that she derailed Anita's entire character so that she could have sex with her husbands SI.