You know, I didn't like it at the time, but in retrospect - I totally see Mukti's reasons for doing what she did that night. At the time, as a woman I wanted to slap her - sleeping with a guy who was all set to sleep with another woman five minutes before her...it seemed so degrading. But having looked at the bigger picture - Mukti saw something in Prem that no one else could see, because he wouldn't let them. She gave him what he needed (but would never admit to) - love.
That is also so in character with the old Mukti. Charging in, with total trust. And I completely agree with you abt Prem. His point of view was right in a way. He didn't love her. (or didn't know it.) He didn't pretend to her, he didn't try to deliberately hurt her, it was just how he felt.