Lets think like her for a few minutes

TL:DR She will cheat on you. Your attitude in life will judge whether she fucks you again after that.

I'll be making some wider generalizations here, but bare with me because they are all probable likelihoods, becoming more probable the hotter your plate is.

Lets think about what it's like to be a girl. The world revolves around you. People are nice to you. If you're hot, you get pretty much all the attention and anything that you're needing without putting in any effort. Even if you're a bitch, people still treat you nicely, and if they don't, the white knights will pop around the corner to defend your honor.

You're sleeping with a total stud. Mr. Alpha himself. You're just his plate, but that's good enough, he makes you feel amazing. He's out on business, you're on a night out with your girlfriends. Comes along a guy who oozes sexuality and confidence. He looks at you like he's looking at a child, makes a few sly comments, closes into your personal space, and all of a sudden, you're feeling your vagina burning and your panties getting wet. He casually asks for your number and to meet up with him after you've said goodbye to your friends.

What could go wrong... it's not like your Alpha stud will know. And it's his fault, he's not here to satisfy your urges... And who is anyone to judge when and where you will find your satisfaction. All your life you've been given everything you want, right then and there, in the moment. This is no different. You fuck him that night.

Now lets change the story for a more beta boyfriend. Or even not a boyfriend, lets consider the story in the case where she is the plate of a guy who she knows likes her, but won't commit (in his mind, he's reading TRP, and trying to plate her, but he lets beta tendencies show). The story is exactly the same, she sleeps with the guy.

For this kind of girl, there really is nothing you could have done previously. If she's sure nobody will find out, in her primitive mind she's risking nothing, and she will cheat. What does differ is the aftermath.

For the Alpha:

Your Chad comes back home. He calls you over for a fuck. You're ecstatic, you go to his place, he fucks your brains out. Chad #2 who you had fucked is just that, another good fuck in the back of your brain, but who cares, this is pure sex, and you're getting fucked by a god among men. The past is the past. It's not like he would care anyway, he probably fucked lots of girls on his trip. But of course, it's your little secret. Still, it doesn't grow from there and doesn't take over her hamster.

For the Beta-turning-Alpha:

Your Beta-Alpha gets home, calls you for a date. You know he likes you. He's a nice guy, he's good in bed, he even turns you on at times and can be manly. But you remember that weekend, where you were fucked as if you were a latex doll, no regard for your safety of pleasure, and how you had multiple orgasms that were much more intense than your Beta-Alpha boy gives you. You feel bad that he can't do that for you, it's not his fault, he just doesn't "have it".

This right here is the seed of disgust. It has been planted. Every time she sees you, she will more and more compare you with that Chad she fucked. When you try and get sexual with her, she won't know why, but she'll feel disgust. She doesn't want to feel your dick inside her, because it feels inferior to what she deserves. In her mind, you become needy, she wants you to be strong and stoic, just like that guy was. So she'll argue, she'll bitch, she'll sabotage what was just a simple FWB relationship because you invoke feelings of disgust in her.

Basically, showing signs of neediness and signs that you really like her will destroy a FWB relationship. When she fucks another guy (she most likely will at some point), and starts thinking how much that would hurt you, her natural reaction is to hamster the blame on you. And the first thing that happens is she starts to grow disgust for you. And then the sex dies.

Lesson Learned:

Always show less interest in her than she shows in you. As long as she's more invested, and you appear less invested, no matter what she does on the side, there will never be that seed of disgust. That seed comes from her hamstering "hurting you" into "disgust for you", and it's a natural way women get rid of men they are no longer sexually attracted to. It's nature's way of ensuring the inferior sperm doesn't fertilize her. Disgust for sex.