nija'd me, doublecross
A comatose person can be dreaming. More importantly, a comatose person has a personality, a sense of self, has been conscious, and could return to an already existing life/personhood. A zygote is not a person yet.
As for the idea that it might be morally wrong to convince a close friend not to have children, I would take it a step further. If it is wrong to prevent a future life from becoming realized, what about making the decision not to constantly have sex at every possible opportunity? After all, this violate the biblical commandment "be fruitful and multiply." That would also be morally wrong if we accept that argument.