You're not as smart about this as you want to believe, pimp. You're treating single mothers like they're some sort of sub-culture, which, c'mon fam. I was always open to dating all women much like you seem to be. That included single mothers. But at no point was I doing that because it seemed easier. They were women that I liked who just so happened to have a child. It was never a dating ponzi scheme.
But, since you have attached some harebrained ethos to your dating preference, let's talk about
this idea of treatment you have created in your head. Treating a single mother better than her baby’s father is not just the least you can do, it's less than that. The bar for treatment of any woman, with child or not, should be as high as you can possibly make it for yourself. Stop assuming that the person who hurt her most or mattered most in her life is her baby's father. The reality could be some of her deepest, most life-changing relationships were not with him. You need to be better than all the men who came before you, not just the father of her child. That is of course if you actually care about not meeting the same fate as those men, and judging from this question, I doubt that.
Something about your question tells me that these days you're only dating for sport. That's perfectly fine, but you can do that without making the type of women you like to date feel like you're doing them a favor. No one would say you're wrong for being willing and even preferring to date single mothers. The issue is, you think when a woman has a baby and ends up single, she’s lowered her standards, and what’s even more sad is, you have somehow convinced yourself that you are this lower rung of man she is seeking.
People that tell you you're taking advantage of single mothers may be right, but for the wrong reasons. Any time you try to put your dating preferences on a pedestal as a way to make yourself look like a hero, you look like a clown. Some women may not see the jig, so in that sense, you've successfully made yourself out to be this noble guy when really you're being selfish as fuck. You're like the white person who wants black friends to show they're not racist. If you want to date a single mother, do it for the same reasons you would date a woman who doesn’t have a child: You dig her company, she turns you on, and you enjoy being with her more than anyone else.
You think you’re the only guy in the world who is willing to date a single mother, Christopher Columbus? You’re not. Plenty of men don’t care that a woman they’re dating has a child, but the smart ones know better than to act like this is a niche that's easy to exploit. That's the ironic thing. Smart men know that dating women, whether they're single mothers or not, is never easy.