My told me when I was young, "The wife/girlfriend etc is right, even when she's not." I didn't realize what that meant until I actually met my beautiful fiance. It's ingrained in our culture; men are the stronger protective gender, and it's his fault if something goes wrong. Facts back this up for the most part; 95% of all relationship abuse is husband/boyfriend on wife/girlfriend. There are far more single mums than single dads.
I'm not saying I totally agree with the 'man is always wrong' assumption, but more often than not, it's true, sadly.
If a guy leaves a mother with children, he should try to atleast get joint custody. If he doesn't even try to take care of his kids or the woman he left on her own, hes a horrible person.
I grew up with a single mother. It is not easy. She had two kids to take care of and could barely make minimum wage, struggling with drug usage, etc. She had to work two jobs just to pay rent. Luckily my family took good care of me. Without them I'd probably be dead or homeless. My dad couldn't do anything because he was in jail until recently, but for someone who leaves their children voluntarily, they're scum.
It'd be different and if there was no children, I could see it then. But trust me, being a single mother is one of the hardest things ever.