Some snap judgements here.
Is OP a woman? Could be a man.
Anyway, as someone who was left pregnant and with a toddler, I started online dating when my baby was still months old (and breastfed shock horror!). I hope you don't judge women who are single parents in the same way 
.
I have had 2 boyfriends in that time (5 years). First one I dated for 8 months, he had a 6 month old when we met. His ex left him about 6 weeks before the birth. There was never any problems with co-parenting. He had his daughter every weekend and for long extended holidays from her being 12 weeks old. The week following our first dates he had her for 9 days whilst mum and her other daughter went on holiday skiing. Anyway, I ended that relationship because it just didn't feel right and didn't think he was the kind of step dad I wanted for my kids (kind enough, but my kids needed a father figure, not a friend, as their dad buggered off). Not all men who are single and fathers of young children are awful men or partners! He would definitely have made someone a lovely partner. I wish him nothing but happiness. He just wasn't the right fit for me.
My current partner also had a 6 month old when we met. His ex left him in early pregnancy when they had only been dating for 3 months. He had been single for over a year when we met. We have been together 3 years now. He is a lovely man, and everything I could ask for. It hasn't been easy as his ex didn't want a dad for her child. She has very much tried to keep him away. She has been unsuccessful in this, but that has not been without its stresses.
The actual step parent side of things has been fairly simple. My boyfriend does all the parenting 99% of the time. I look forward to when she is here, as although it is harder work having a 3rd little one here, we have lots of fun together.
Anyway, stop with the snap judgements to those who want to try them. Not all men walk away from young families. My ex might have done so, but I'm not going to judge every man by the actions of some.