Spectre,
As was clarified previously, the passage about Onan is not about masturbation, but rather withdrawl before ejaculation to avoid providing an heir for a dead brother by impregnating his widow.
The idea that masturbation is morally equivalent to abortion is not found in scripture. Certainly its not the case for female masturbation.
For your consideration:
The original question has three parts.
1) If you are into your religion,
2) is it a sin
3) to masturbate?
The question does not specify which religion, so anyone may reasonably respond from any religious perspective. That the discussion seems to presume a christian framework of understanding is quite limiting, don't you agree? I would love to hear from others who hold different faiths.
The question asks about sin, so in order to answer, sin must somehow be defined. In some religions, the concept of sin may not exist, or sin may be understood in ways that are different from Christianity, Judaism or Islam. In a religion that does not recognize sin, masturbation cannot be a sin, yes?
The question asks about masturbation, but clearly from the diversity of opinion about what that means or what that must necessarily entail, even this part of the question must be clarified. Some people cannot imagine masturbation without fantasy involving another person, so for them, since lust is always involved, perhaps even by definition, masturbation is sinful by virtue of the lust. For those who can masturbate without such fantasy or lust, what then?
As for atheists, in the classic sense of the word, I enjoy discourse with them for I find they have reached their positions through principles and reason, and they challenge the faithful to examine their own assumptions and reasoning, something that is desperately needed these days. Certainly anyone may quote scripture, but my hope is they do it with flare
Peace,
Parable