I don't think the question itself is bad and it can be sweet. Never the less, it is a delicate question since the answer is very personal and there can be many painful reasons (being left at the altar, anyone?).
I am glad you two are happy!
I was thinking much the same reading through this. I'd never call the question presumptuous or wrong - in fact, it's a common one - but on a first date, nobody should expect full disclosure. So, OP, your only problem is which of the great suggestions you use and *when* you use them.
For example, I've only had 2 serious relationships
in my life, both long term: My DH and the guy before him. I can paint the guy before him in glowing terms, because he was/is a great guy, we had a forced separation of over a year due to his job, that was tough on us and things didn't seem the same when he got back. That's all true, but it's the sanitized version.
The factoid left out is he cheated on me during that year. During my dating
days after this, I reached a point where I could talk about it, but never on the first date. It was too personal.
So I can see where a fairly innocent question has its share of minefields attached. But you get to pick what gets disclosed and when. And this all sounds positive for you. I hope it continues!