That choosing a relationship with someone who is monkeybranching into the relationship with you directly from another relationship is you allowing someone in your life who is fundamentally dishonest and manipulative. It's one thing to be casually dating in general, and just finding someone you click with and ending it with the people you are casually dating, but entering a relationship with someone who pursues you even though they're in an ostensibly committed relationship is choosing to accept someone who is really not a good person, because they will just do whatever they want and eventually hurt you without a qualm too. Tolerating any of this means you are tolerating abuse, really.
Unfortunately he didn't tell me this fact until 18 months into it, but that should have been what made me realize that he wasn't trustworthy and leave then.
Also committing from the get go and falling in love? That's just also not valuing yourself. You're just looking for someone to fit into your life because you don't love yourself enough to wait and take your time and get to know someone, and you're afraid to be alone and have nobody to care for you. And I did all of that, because I was immature, completely without any idea of how to make it in life alone or cope alone, and I thought that was all I deserved and was the only way to be safe. And it was all wrong.