I'm going to add in a little 2 cents that might counteract some of the teachings, yet they're based on them.
My main plate is also seeing some other guy, but only having sex exclusively with me. I was fine with this until quite recently, as I discovered the guy is a total douchebag. Since I'm quite into this girl, she's an 8 / 8.5 in my opinion, I was done with it, and told her to make up her mind: In a short undefined timeframe, she'll has to make up her mind about this. It's either my way or the highway, and I specifically told her that I will walk away if she does not make the choice, which includes telling the other guy that she's seeing me exclusively.
The next day the calls up to ask if she can sleep over, and when I ask her to join me in the shower, she told me she already showered at the gym before coming my way, and when I return: She's in my bed in a corset and stockings. Next weekend, she's in a club singing along with Eurythmics - Sweet Dreams and during "Some of them want to be used by you", she's all over me and whispers in my ear the other dude is out.
It all comes down to laying down your law. Be decisive, be strong willed and have the guts to turn around and walk away.