In my experience single moms usually fit 1 of 2 categories with very few exceptions:
1) She hooked up with a guy that had a few flaws, but she found him exciting and figured she could fix him. She may or may not have married him, but she just knew he'd make a great father (never mind that he kept saying he didn't want kids), a couple of kids later he dumps her or she leaves him because he wouldn't give up the drinking, pot, other women, etc.
2) She met a guy and had a kid with him thinking he would marry her and big surprise, he split instead.
It takes a special guy to be a father, the responsibility is awesome and fatherhood should be entered into freely and without coercion. The cure for the epidemic of single motherhood will come with an effective male birth control pill. Once that happens, every child born will be wanted by both parents.
Single mothers tend to be serial offenders and if you hook up with one you'd better realize that you'll never come first in her life.
.....and no I'm not bitter, just a realist.