In Sweden, we have many terms for a man dating a single mom:
bonuspappa (bonus dad)
plastpappa (plastic dad)
extrapappa (extra dad)
E.t.c..
Dating a single mom is not stigmatized at all on Sweden, probably because of our high divorce rates. Many of my classmates that I grew up with had divorced parents with new partners and new children. This creates more dynamic family trees with many half-siblings (kids sharing one paternal part)
I think dating single moms should be even more accepted, they are experienced, nurturing and know what they want.
It's ok to date a single mom, it's better for the children to grow up with a fake dad, rather than not having a father at all (check out the crime statistics for single mothers).
Stop the stigma