When I approached my husband, I was prepared for him to say he wasn't alright with me pursuing this new person in my life. I feel strongly that I have the desire and capacity to love more than one person, but I was, and still am, entirely willing to compromise with my husband if he decides it's not working for him.
I will be heartbroken if he asks me to stop seeing my boyfriend, and it will be hard, but this relationship is not a dealbreaker for me. It's different for each person, I think, but I'm unwilling to sacrifice my marriage, and I know I can be quite happy in a monogamous situation. That's not the case for everyone, but I'm not so poly-wired that I can't remain mono with my husband if that's what he wants again.
I won't, however, give in without a fight. It meant a lot to be able to pursue my new relationship, but if my husband changes his mind, we'll talk about why, and it will be a decision we make together while both hearing each other out.
Your question is valid, but the fact is a lot of poly people cannot be happy in a mono situation. In that case, the right thing to do is often to end the relationship so each partner can seek a better match.
I firmly believe no one should be or feel bullied into being someone they're not on either end. Period. Compromise perhaps, but when one partner feels marginalized or bullied, that's not healthy regardless of the arrangement.