Her fears are that I won't love her anymore, abandon her, and make her feel cheap through this. None of that would be a reality if I'm with Penny or not and there's nothing about my specific proposal that would facilitate that. If anything, there's several measures to protect against those things. The problem is that she's not really looking at those as much as she is her initial gut reaction.
Her fears are entirely justified…to her. You are choosing not to see things from her perspective, just as much as she is choosing to not see it from yours. So let's break this down.
1) She worries you won't love her anymore. You think this is nonsense because you do love her. But if the way that she FEELS loved and the way in which she receives love (and possibly the way she gives love too) are tied up with sexual and emotional exclusivity, then she is entirely correct. You dating Penny will unequivocally be demonstration of the fact that you no longer love her in the special way in which she is used to being loved. People express and receive love in different ways. She is clearly telling you that she won't feel loved by you anymore if you pursue this course of action, and instead of hearing that, you are trying to argue the toss with her. At the end of the day, it doesn't matter how strongly someone feels love for me in their heart, if they never tell me, never show me, never communicate that to me in a way that I can understand, then I'll never feel it.
2) She worries that you will abandon her. You think this is impossible because you have already decided you would only see Penny when Mabel is at work, and possibly occasionally for a day or two over the weekend. But she's about to have a baby. She's not going to be at work on a Wednesday, and if she is, then presumably you are going to be left holding the baby. There's nothing romantic and sexy about being covered in vomit, so you are going to have to find time outside of that to be with Penny. So you think to yourself, well, I'm only gone for a night at the weekend, that's not much, right? Wrong. There will be times where she needs you and you will not be there. Maybe the kid runs out of nappies in the middle of the night. Maybe she feels unwell. Even if you hop in the car immediately and drive home, you are out of state. You are not going to be there when she needs you. And whilst maybe that's not your definition of abandonment, you cannot change the fact that in taking on a new relationship when you should be wanting to throw your full weight behind the most important new relationship of your life (the one with your child), you are spreading yourself thinner than you ever have before. In her shoes, hell yeah, I'd be worried. Add in the fact that you're new to poly (so will fuck up until you learn better), new to parenting (so will fuck up until you learn better), and that you and she weren't doing so great with each other before this fiasco started (so you have fucked up already, and are still figuring out why and how to fix it), and I'm not surprised she's refusing to even entertain the thought of this.
3) She worries she will feel cheap in all this. You don't see anything shameful about sharing your partner with another person, so you think this is silly. But again, you don't get to change how she feels about a thing. You also don't get to control how other people interpret this. If her friends and family, upon hearing about you and Penny, start to pity her, there's nothing you can do about it. If people on the street start to gossip and THEY think you're stepping out on your wife because she's not satisfying you in the bedroom, there's nothing you can do about it. But those things are going to impact her, and the way she feels about herself in a myriad of ways. Even if you all keep this whole thing a secret (not recommended, by the way) it sounds like you having another girlfriend makes HER feel like you don't think she's enough. And the hard cold truth of it is, she might not be. Rather than deny that she should feel that way, why not acknowledge the truth that's there? Yes, other people may think less of her because of your choices. Yes, she is not enough for you, but then no one is, because what you crave is variety (or whatever your actual feelings on this matter are).
In short, dismissing her fears as trivial when they actually have a sound basis is a douche move. I think the saddest bit about all of this is that instead of falling in love with your new baby, bonding with Mabel, and enjoying all of the possibilities the future might hold for your new family, you are mooning over someone else. Your comments about how you would take the baby off her hands, and that Penny was fine with kiddo coming over with you sometimes, suggests you may have completely missed the point of what this journey into parenthood means to Mabel. I bet if you asked her what this baby means to her, you would find that she doesn't just want you to take the kid off her hands now and then, but for the pair of you to raise it TOGETHER. The idea of another woman, a woman she may well dislike, looking after her kid is NOT the kind of support she needs. It's a baby, not a bloody time-share.
