Regarding Catholicism, the whole of the human is created at the moment of conception, fertilization of the egg. You might be thinking of that Monty Python skit, 'Every Sperm is Sacred'. Masturbation is considered a sin because it intrinsically involves lust. You are correct about it boiling down to "when does the soul enter the picture?" Catholic answers to this question were further defined by Augustine of Hippo and Thomas Aquinas. For the SparkNotes version, look at the Catechism of the Catholic Church. It's a little dogmatic, but it's based on Natural Law.
To simplify: over time, if the situation begets life, it should be promoted; if it does not, it should be discouraged. That's why Catholics defend a frozen zygote, as opposed to a comatose patient on life support.