D is not opposed to marriage or children but I am.. because I am afraid there relationship will grow into something I am not comfortable with or the gf will love D but not me or the gf will behave differently when I not around. I would love to get past these issues I feel I can but am afraid I will be second in D's eyes and the gfs
Well, the chances of another person loving you both equally is about zero, so you aren't wrong to think that could happen. It most likely will be the case that one of you will be more emotionally involved than the other, and that may not be you and she. Or, it might be. That isn't the end of the world, but if it's not something you're open to, then poly isn't for you.
What you are looking for is called a "triad," and also sometimes called "unicorn hunting," because you are looking for some very specific things. Before you go any farther, you should read
this article. Really, really read it. It'll help explain a lot of what can happen, and likely will happen, in the type or relationship you are looking for. Then, you can decide if you are open to that, how to best deal with what may come if you do still want to proceed, etc.
It also seems like you and your fiance may want, or be open to, different types of things. He is open to marrying and having children with someone else, but you are not open to that happening, and want to be "first" in his life. Before you go any further, you both need to figure out how to be on the same page with this.
Also, and no one likes to hear this, but it needs to be said: closed triads (meaning three people who are involved with each other, but with no one else) are notorious for ending really badly. Poly itself will shine a light on every single crack, insecurity, or problem in a relationship, but triads will do it in a really crazy and special way that can leave a lot of people very hurt. I am not saying triads never work, but they rarely work when they're sought after specifically. Which is why you'll see a lot of advice here is to date separately, and see what happens--triads that form organically and naturally have a better chance at working and not ending in fairly horrible ways.
In terms of GalaGirls question, you should try to be more specific. "Emotions" and "attention" are very vague, and your partner(s) can't mind-read. It's also a bad idea to try and legislate emotions. So, if what you mean is your fiance has to "love you more," or something like that, poly is probably not for you. Emotions don't work like that, and trying to force them to can destroy relationships. As far as actions, what actions? Maybe something like you need to spend a certain amount of time alone with together with him, to help keep your relationship growing? Or, maybe you need physical reassurance such as hand holding and snuggling? Or, maybe you would like to make sure you and the new girlfriend have a certain amount of alone time to build your relationship?