Being lonely in a relationship is far worse.
I think previous relationship history does play a factor ridiculously. In the last few years I dated someone who had never been married or had kids and his longest previous relationship was only 2 years - I soon found out why - he was emotionally shut down, had no feelings, it was all practical, physical, logical and cold, if I'm going to be in a relationship rather than a casual sex thing then I want some feelings somewhere. Another had a few long term relationships and kids and again I found out why - he's a serial monogamist desperate for a happy ever after & true love and it doesn't matter who with as long as it's long term, committed, very heavily embroiled and intensely one on one, I want someone compatible, not just willing to commit!
On the one hand I want someone who is used to kids, knows what it is to look after them properly, put them first and be a proper parent (though having kids is no indictation that they are any good with them as I have learnt the hard way!) but then I don't want my daughter feeling pushed out or jealous that someone else's child is getting my attention so I would be open to a father or not. But yes at this age I would be wondering why they've not been married or had kids and if they'll really be any good with my kids and understand them and the boundaries of a step parent relationship as compared to a parental relationship, have they been a player in which case I do not want to be the first commitment they've had, I'm too old to be their experiment finding their way in a solid relationship, and I do not want to have to deal with hiccups and mistakes they'll make with my child if they're not used to kids or the battle of them having to learn they are not my priority and my kids will always come before them.
I agree with those who have questioned whether their friends & female relatives are really happy as I look at the men in my girlfriends & family's lives and I think they are not good enough for them and do not treat them properly and I would not stand for their attitudes and actions.
Maybe no one is really happy, maybe people just think staying is better than leaving. Maybe I do have unrealistic expectations of happiness, confidence, empowerment and respect within a relationship. Maybe everyone does have a deal-breaker but people just put up with it regardless? I dunno. I'd still rather be single than putting up with bullshit.