Matt: I have enjoyed your posts. I believe you, no one would make up something this goofy.
Here is my advice, its free and worth every penny of it. On Monday night you should have insisted on sex, she may have been sore and tired, but one more roll in the hay would not hurt her. She assumed that you had been celibate for two weeks and should have been horny. Then she found the note in your pants and heard the story about you kissing at the bar. Forget about denying what happened, she will never believe you (not after what she has been doing). Based upon your account, I suspect she likes forceful men, try to be a little more demanding and forceful yourself.
Also, if and when she calms down, tell her that you cannot survive on "after thought sex." Most of your sex seems to be after she has had a night of sex with someone else and your sex is more charity. Tell her you need an occasional weekend with her for fun and lust. She can just tell her friends that she is going away for the weekend without any details, doesn't she want to be known as a free spirit?
Tina should marry you. Her other boyfriends may enjoy a night or weekend with her, but none of them would put up with what you are going through.
Finally, there is an old adage that applies to you: be careful what you pray for, you just might get it. Has it occured to you that she might marry you some day, and then decide that she has had enough sex for one lifetime and the two of you can be celibate. I do see one hope for you: when you I die I am sure that the Catholic Church will canonize you and make you the patron saint for unrequited lovers.
Good luck, and think long and hard about what you want.