Disclaimer: If you enter into a committed monogamous relationship, you owe it to your partner and yourself to remain that way. If the relationship goes sour and it's not worth saving, then move on. Open relationships or non-exclusive relationships are just that, open.
It happened to me twice; I got involved with a married man. I say it that way because I didn't know they were married when we first got involved and when I found out, I didn't move at lightning speed to end contact with them. I was pissed. They weren't honest with me like I was with them. One waited until things got hot and heavy and then decided they really liked me and should tell me the truth. After that experience, I've been on guard. So the second guy, I had to ask. He wasn't going to tell me. When I did ask, that scared him off. He didn't answer the question just walked away, literally. Two things he wanted to know as he was walking out the door were, was I going to kick him and if I was going to protest (meaning stop having sex with him). Looking back, I think he felt that was his only recourse in what he perceived to be a power struggle; end it with her before she ends it with me (this is my hindsight insight).
With the first guy, I stopped seeing him. I'm too selfish to share. I'm Queen Bee, not someone's side piece. It's taking him a while (6 months) to understand but he has no choice but to get it. I don't know his wife or kids but some mistresses who are involved with married men want to know everything. They want to develop that emotional bond, be the one he confides in and does all the freaky stuff with then when they sense him pulling away, they threaten to tell their secret. That's what gets them stabbed in the neck. You do not have the power in that situation. A married man will not leave his wife, family, home for his mistress no matter how rocky things are at home. If the mistress starts to demand more of his time and energy, expect the affair to be over rather quickly. If you choose to "stay in your place" be prepared to be available when he has time for you. Whining and nagging about why you didn't hear from him over the holidays or your bullshit anniversary will get you nowhere. You are not a priority when he has family obligations. The power of the pussy is strong, but it aint that strong when he can find the next woman who will keep her mouth shut.
You are mistress, not master. Next scenario, he throws around the words "I love you", "I need you", "I love spending time with you" (a good way to slip in the L-word and blind certain chicks). For some women, those words are like a tether wrapped around their neck. They can't walk away once a man says that no matter the situation. He's in control. And please don't get pregnant. Your love child isn't going to mean anything. You may get money from him; part child support, part hush money, but you won't get him. How often have we seen women with the bastard child and the man admitting his years-long moment of weakness, with his wife by his side?
Master or mistress?