Thanks for asking this question. Being a single founder is hard, but doable if you are passionate about what you are building. If you aren't passionate to the extend of being nearly blind with belief, it is impossible to do it alone.
I am guessing that YC may not care that much if you're single or not. I think they evaluate you on your overall chances of success.
Those are some very absolute statements. The blinding passionate belief part is really a myth. You just need to be hard working, committed, and effective. All obstacles can be overcome by focus and reason, not religion or belief. You don't even have to like the work, but it does help if you're not a strongly self-motivated person.
I am guessing that YC may not care that much if you're single or not. I think they evaluate you on your overall chances of success.