Originally Posted by Confused22
I have told her that this simply will not work, it's not something I feel I could ever be okay with, and I'm not willing to try it and see.
"I don't think I can remain faithful to you anymore"
I love her dearly however. We've been through hell with, and for, each other. We live together, 3 years now. We have a son. We have a happy home and family. Why does she seek out to destroy what we have with this? Why does she feel like I should have to share her? I didn't share my Legos as a kid, and I am sure as hell not inclined to share the woman I want to marry.
I will confirm what others have posted - it's not a good idea to launch into a poly or open relationship when the original relationship is flailing. Your post hints that there are lots of things going on beyond the desire for poly. Try to address and resolve the issues between you. Do this if only to make sure that if the romantic relationship does not continue, then you two can be good co-parents for your little son.
Also, your girlfriend, the mother of your son, your lover is not a Lego. Not a clickable bit of plastic you attach to other bits of plastic. I too didn't share my Legos as a kid. Hated other kids even touching them. But people are not possessions. You don't own her. She doesn't own you either. You only get a say in who she 'shares' herself with only as long as she says you do. Bluntly, it's not your decision on when, who and if she shares herself with. It's hers alone. The consequence of that decision might be that your romantic relationship ends. But it is her call, and yours in how you decide to react to her decisions and needs.
There is absolutely nothing wrong with wanting monogamy. There is nothing wrong with wanting to be the sole focus of your partner romantically, sexually. It works well for lots of people.
If that is what you want, what you need, then end this relationship gently and transition to becoming co-parents. You've pretty much stated that you don't want a poly partner and could not tolerate her being poly while you remain mono. (Some people make this work but it is a hard road.) You have a son together - you will be in each other's lives regardless of how your romantic relationship works out. Focus on remaining good co-parents for your son.
This is painful stuff and might be a fundamental incompatibility. That is hard and I feel for you.