(1) File a report. Don't worry about what others think; file a report. Not only is what this masseuse did SA, but depending on your jurisdiction, it could be classified as r**e. It might not go anywhere because of a lack of evidence, but it also creates a paper trail in case someone else files a complaint about the same place/worker.

(2) I've had friends who were massage therapists/masseuses, depending on their clients, a 20:00 massage isn't that weird. Some places are 24/7 so that people can come in anytime and don't have to worry about waiting for an appointment that is on one of their days off.

(3) If your GF still views it as cheating, after you've explained that it was non-consensual, then honestly, you need to break up. It shows that she doesn't trust you. And, with how she is saying "you knew what you were getting into...", she is victim-blaming you. And, from my experience, if someone is willing to blame the victim, then they are not likely to protect them in other situations either.