If you think for someone who's a parent that becomes their sole defining characteristic from that point forward then I can't help you.
Being a single parent is also just a negative in the way you chose to define it.
Again, I dated someone with a kid, it wasn't fundamentally different than most other relationships I've been with. Sure it had it's own challenges, but everyone comes with their own limitations, strenghts, weaknesses and particularities. It's like saying "I won't ever date a gamer because I don't want to deal with someone who has shelves of games and cares more about them than me" you are basing it on your own assumptions of what being with someone who is a parent really is, which is unfair.
I think it says a lot about a person if they write off a whole diverse group of people from their dating pool, when that group of people has a lot of difference among themselves and can be a number of different situations some good some bad.
But yeah, people are free to do whatever they want, I don't really care.