First is mommy's new boyfriend. But possibly because numerically that is more common. There is a reason "evil stepmother" was a thing in folklore.
Am I saying that if you are a single parent you should not have any sort of romantic relationships until your kids are grown? That is up to you.
But am I saying that unless you are willing to put your kid before your current bed warmer that you should sleep alone? Yes.
I married into a family - but I knew, and readily accepted, that the kids always came first. I even insisted that she stay home with them and raise them like a mother should because I had been through the baby-sitter plan and wanted nothing to do with that for my kids.
I totally agree. Mom or Dad needs to prioritize children before lovers. Plus, raising kids is hard. Who has time for a love life?