I have rheumatoid arthritis, and I've had it since I was a child. (Yes, that does happen.)
When my now-DH started getting serious, I sat him down and walked him through it all. How it had progressed, how the future looked, the risks to me and future children, the uncertainities regarding pregnancy, etc. I told him I understood if he didn't want to committ, because of the burden.
He immediately said, "I love you. All of you. That stuff doesn't matter."
That was 15 years ago. We have two fantastic kids (one with rheumatoid arthritis), and he has never questioned it.
Sure, he does have to pick up the slack sometimes, or teases me when I hobble on really bad days, but my RA has never been an issue to him or our relationship.
He's a keeper.