I suggest you take a good look at the situation, and more specifically your relationship with him.
He's been with you for a long time. You start a new relationship, and this other person gets to move in with you. This other person is who you propose to. And on top of that you say you prefer females, if he knows that he must be feeling extremely neglected and replaced.
You need to give him more importance and support. You talk about two primary partners, but the way your posts read, he's become a secondary partner now. Not even taking part in big decisions, only getting told after the fact.
I think you need to tell him, and simultaneously apologise for treating him like that, promise you won't do it again, and keep that promise. And from then on, make sure you treat both of them equally, as important to you, rather than involving her in the big decision and telling him after the fact.
The longer the wait, the worse it's going to be. And it will probably be a long talk, because I believe you'll have to talk about the whole relationship. Don't walk into it thinking you were justified and that he's going to overreact. Walk into it knowing you didn't give him the respect he deserved, and that he has a right to be upset, and that if you want it to work you will need to rebuild trust with him, and that means giving him more time and attention, and making your relationship with him a priority over your relationship with your girlfriend/fiancÚ - for the time being. There is more work to do with him and therefore you should be working on that relationship more, or it will keep degrading until it's broken for good.
Also talk to your girlfriend about it all, make sure she understands you love her but you fucked up. Hell, have a big talk with all three of you together. You're a family now, it's not you and him on one side and you on her on the other, it's the three of you, even if they're not involved they're still part of each other's lives and will be at least for as long as the relationships last.
Now, my other question is, what about him moving in? Is that something you discussed? Is it something you're not interested in? What about getting married? Is it something he didn't want to begin with, or something he'll be hurt that he lost to her? Is having a ceremony with him an option down the line, even though it would have no legal ground?
Right now, it looks like your relationship with her is eclipsing the one with him. They don't have to have the same rate, or follow the same steps. But you need to make sure not to neglect him for her.