Captain Fantastic pretty much summed it up in small words that you should be able to understand, even through the cognitive filter of wishing things were the way you wanted and a stiff dick.
It's about boundaries, we all like to think that we'd do the right thing, no matter what. The reality of that is though, that most people will only do the right thing until it becomes morally or financially difficult. She didn't go around pulling off one night stands, she waited until she spotted someone who looked like a good potential SO until she let her panties hit the floor. She didn't bail on the relationship she had the day before she spread her legs, or even the day after.... noooo.. because then she wouldn't get the nice vacation!!! She's got morals and she's got boundaries.... as long as they don't get in her way of having what she wants..
One day, being the one you would like her to be will be incongruent with "what she wants".. and you'll be the cuckhold asshole dishing out for the vacation while she wipes off the millions of potential children that died on her chin last night, that weren't yours, before she kisses you goodbye.. She has shown you exactley who she is and how she rolls... Ask yourself this.... If your younger, not too bright and a bit naieve brother asked you for advice on this situation, what would you tell him to do?