Some of us can decouple copulation from the consequence of pregnancy. I know you're talking about our (British) society, but there are millions of people who can't do that uncoupling yet. In those (human) cultures I don't suppose there is much opportunity for women to choose to be a single mother. It is probably frowned upon, as it used to be here, sometimes with very nasty consequences.
I don't know anyone who has chosen to be a single mother though I do know several people who became single mothers for various reasons. I don't think it is all that common, is it?
Perhaps it is selfish but so are a lot of other things people do. It's not a choice I would make (at least I don't think I would have done; too late now) but I don't feel like condemning others for it. We cannot know what has made them make that choice. In some cases, if we did understand the whys and wherefores, we might feel quite sympathetic.
Other apes, elephants, whales and cetaceans have more cultural mores than are just to do with food, I think. There could well be other species too – wolves and wild dogs, lions..... The food ones are important, as they are for humans, but they are not all there is.