Hi Raena.
I am sorry you're finding yourself in this position...I honestly can't imagine what you're feeling right now. I've read a few stories similar to yours, and each and every time I try to put myself in your position mentally, it just...doesn't work.

I virtually cannot imagine my reaction whatsoever.

Regardless, I am sorry you are going through this.
Raena, this might be hard advice I'm about to offer...but I would tell you to go with your gut and your heart. You obviously are not comfortable with the idea of sharing your bed with someone else, and he is obviously very set on the idea.

If he's not willing to budge and neither are you (which I don't fault you for at all...I wouldn't), that's a very significant impasse that needs to be addressed, you know? You need to do what you are comfortable with...never, under any circumstance, commit to something in a relationship, be it sexual or otherwise, that you are not comfortable with. This is certainly no exception.
To insert my own opinion...I've nothing per se against open relationships. The idea is very, very alien to me, and I cannot wrap my head around the idea, but I don't pass judgment on those who are in them. However, despite my lack of understanding there, I DO know that it HAS to be agreed upon and be comfortable by both partners. BOTH partners. Respectfully Raena, I don't think this is something that you should really have to "try to be more open minded" about. To me, based on what you've said, this is obviously uncomfortable to you.

I respect your dedication in trying to compromise on that, I do...but if it's something that truly bothers you, I don't think that's something you should have to compromise on, you know?
Ultimately, call me old fashioned...but I think if one of the partners in a potentially poly-amorous relationship is not comfortable with the idea, then the idea should be dropped or the couple needs to reconsider things heavily, if you take my meaning.

I am sorry...I hate advising that.
Please know you're in my prayers, and I wish you all of the best.
Hugs,
Harley