How is adoption selfish?

I myself have never had any personal experience with adoption so forgive me if I offend any of you but what I'm noticing lately is that a lot of people are becoming anti-adoption. Now in a circumstance where a girl who is willing to sacrifice whatever she needs to in order to parent and give her child a good and stable life, who is bullied and coerced by her family members or adoption agencies into surrendering her child, is not okay in any way, shape, or form. But in a situation where a mother does not want to take care of the child (not can't, won't) how is her giving her child up for adoption selfish? Why should she have to mother the child? Yeah, she should have thought about that before having sex but in reality contraception fails, and telling people just don't have sex or don't get pregnant usually doesn't work. I mean either give the child to people that want to have a child and can take care of it and most importantly love the child, or have the birth mother take care of it and possibly give 18 years of hell and lifelong problems? I know some children who were adopted have issues because of their adoption and their birth parents, but I still don't get why its selfish. Why would you want the child to grow up feeling unloved and unwanted? Why "punish" the mother with a child? Punishment is standing in the corner, a kid isn't. A child should be loved and wanted not tortured because of someone else's mistake. Why not give the child to someone that will love it? (And yes I am aware that adoption won't guarantee the child will have a "better" life but at least it has a chance versus being with parents that don't want it. And don't say "every mother loves their child unless she's depressed or something" that's crap. Tell yourself that if you want to but it does happen sadly.)