Skin cells DO react to the environment and move around. In fact, ALL cells react to the environment and move around. We even use the same word to talk about the movement of cells (including sperm): motility.

But it's a moot point because your conflating LIVING with HUMAN BEING. Every living human is alive, but not every living thing is a human. Cells are living, but the only time you can say that the cell IS a person is if it's the whole person (i.e. a zygote).

It's definitely difficult to feed the zygote outside the womb, but it's not impossible (unfortunately— I don't like the idea of lab-grown babies). But let's go with your premise and say it's impossible. But it used to be impossible for a baby born at 25 weeks gestation to survive. Now it isn't. The age at which a fetus can be born outside the womb, survive, and grow normally into adulthood keeps going down. So do you acknowledge that fetuses grow without their mother, even if zygotes don't? Surely that's a category that's relevant to the abortion debate. It seems pretty obvious that during the growth of the child, it gets easier for someone besides the mother to provide nutrition. Anyone can provide it to a one year old. It's harder with a newborn, but it can be done even before the invention of formula. It would take quite a feat of bio-engineering to supply nutrients to a zygote, but that doesn't mean it can't be done.

Let's go back one more time to the mass murder of sperm to see what happens if we grant your point. Every sperm is a human being, ejaculation is mass murder. The first thing that should strike us as odd is that nature should have designed it so that we had to murder millions of people to make a new human. And then you read that sentence again and realize that the logic has broken down entirely. Murder millions of humans to make a new human? How does that even make sense. We don't need an egg or a woman, we've got the entire next generation already swimming in our loins. If you say that we are killing millions of humans to make a human, you're admitting that the millions WEREN'T HUMANS. They needed to BECOME human. They were "human" as an adjective ("human sperm" as opposed to "whale sperm") but if they were already human, what do we need reproduction for? We already reproduced. Heck, we're reproducing something like 1,500 times a second. And I guess men are murderers whether or not they ejaculate so really it wasn't ejaculation that was the problem, it was sperm production all along! Reproduction would be awfully hard if men didn't produce so many sperm (for many couples it's still plenty hard), but if we just say that reproduction happens at sperm production then problem solved!