For males -
Because it is harmless, probably healthy, male bodies do it naturally on their own, and the “evil” nature of it developed out of the mistaken understanding of human biology. The ancients believed that the male seed carried the entire child and that the woman was merely an incubator. Thus it was very natural to condemn an act which literally killed children.
For women -
Why not?
Of course all this must be based on the commandment to love one another. If the action violates the love bond between spouses, then it is wrong for that reason, not because it is inherently evil. There is no teaching that all selfish pleasures are evil and no reason to believe so.
