Need to be careful with MILF's. They can be a wolf in a sheep's clothing. Their primary concern is their survival, then their kids, then their dog Barney and then everything else. While I cannot tell you that I can confirm that I was ever cheated on, I will tell you, and I've documented here a couple times that when I was dating absolute smoke shows a couple of years ago it was a concern of mine. Two years ago, solid 9's back to back. I went to the hospital for kidney stone surgical removal, and the lady I was dating for five or six months went to another state (airplane) to receive tutoring to complete her masters or something. I just went silent. She came back two weeks later, I recovered and she wanted to come over, and she did. We had marathon sex and a week later she said she wants to continue to see me, but wants to see others too. No problem, take care and have a good life is what I said. Over the next year she apologized, reached out, etc, I was silent. I do not accept that type of behavior. She swore up and down she didn't do anything, but the simple fact that she went on vacation while I was in the hospital was unacceptable.
The more attractive the female, the more interest she's going to receive from other dudes. I remember she showing me texts when we first started dating on how many texts she gets from guys. She even shown me texts from her superiors (teacher) that were married and wanted to sleep with her. I said, then what's stopping you, go if you like, no worries. The point I am saying is that if they feel they are higher than you, they will play the field. You need to set boundries and enforce them. In your situation, just accept it's a FWB and do not get attached. However, I will also stress, if this type of behavior is not acceptable to you, you need to tell her and say I do not like this. You also have to be willing to walk away. You need to have a discussion to yourself on how to proceed with this female and moving forward. I've drawn the line several times, and only once did I actually break my rules and follow up (next day I said I missed her. She then did want to do something two weeks later, but I said, no, this is not right. Take care of yourself. Her only response, I will really miss you. Whatever.). All the others, they came back to check in. I never responded, but it shows conseqeunces for their actions. While you may not benefit from them losing you directly (aside from some self-esteem), the next dude will.