Jools, thank you for posting this.
Im in a similar situation. Though not entirely the same.
But I do not think you are being unreasonable. You seem to be looking at it from all aspects and really questioning yourself, which is a quality alot of folk dont have.
Its important to build a relationship, to build trust and love and all those things that people need to feel secure in a multi-person relationship.
I suppose it depends on him. I HAVE to take that time at the beginning of a relationship to concentrate on someone, even if I'm already in another relationship, I will slow it down to build with the other. I have almost zero free time (full time school and a child who I have no yet integrated into either relationship, and likely wont for a while) and Ive been blessed with amazing partners and amazing circumstances, so it has worked out for us.
If he truly honestly thinks he is capable of establishing a relationship with you while doing the same with someone else, then maybe it will work after all.
If you aren't comfortable, you two can reassess at any time.
Im debating the same issue with my newer partner (who is my primary partner) because he is in a position in his life where he needs a kind of freedom I am not neccesarily 100% comfortable with. However, I understand that he needs to do his thing and we have known and loved each other for 5 years and it's hard sometimes but at the end of the day I trust that he loves me and that he would never do anything I wasnt comfortable with if I said the word.
His happiness is the most important thing in the relationship right now, to me anyways, and if he's happy, and comes to my door with that particularily adorable smile on his face, then I'm happy.
And I'll cross that uncomfortable bridge when we come to it.