I see this constantly appearing in askTRP when a woman cheats on OP.

Man enters a relationship with a girl.

Man has "hot steamy sex" with a girl.

Man is absent for a day in the relationship.

Man is "shocked" when he finds out that she cheated on him.

There are 2 important things that you need to know with regards to women and sex:

1) Having frequent "passionate sex" is not an indicator of a woman's loyalty. While we do say that it is better to judge a woman by her actions and not by her words, having sex with you only means that she is having sex with you at the present moment. It doesn't mean that her hypergamy switch is turned off nor that you have hijacked her thoughts and feelings for the rest of the day. Women have a reactive role in any sexual interaction. The man who she is currently with, is the cause of her attraction, and she is reacting to his attraction. Your passionate sex with her is an indicator of her attraction towards you at the present moment with you being the cause of her attraction. That doesn't mean that you are exclusive to this attraction. Other men can also assume this active role at any moment. If George Clooney walked in front of your apartment's door, while your girl was heading out after a "hot steamy" sex session with you, what would you think would have happened?

2) Having frequent "passionate sex" with you doesn't mean that you cannot be outalphaed. And it should never be an indicator of how alpha you perceive yourself to be. "Passionate sex" is only reserved to whom she perceives as alpha at the present moment. And her perception changes constantly. You see, as I said above, hypergamy cannot be switched off. A BB can have passionate sex if that woman considers him an alpha. It's when an alpha appears and the woman's perception changes that she no longer has "passionate sex" with BB. A lot of times a woman's perception of the alpha changes as soon as she cheats with him, resorting her to crawl back in the arms to whom she cheated. It's not always the alpha's lack of interest that causes her to go back to her previous relationship.

One thing is for certain though, the lack of frequent "passionate sex" means that she considers her SO to be a BB.

Edit: Spelling and grammar

Edit 2: It is my belief that this misconception started from the PUA community, where it says "satisfy her in bed with multiple orgasms and she is yours for life"

Edit 3: To clarify, "passionate sex" is the sexual intercourse in which there is spontaneous mutual sexual attraction between the two parties involved. Not the negotiation sex ("take out the trash and I'll let you go down on me sex"). I have seen it referred to as "hot steamy sex" by some in askTRP.