Is mutual masturbation a mortal sin?
Somewhat. If a man ejaculates, then this is very clearly a mortal sin, as he is spilling the seed. In spirit, the act of mutual masturbation eliminates the loving embrace God intended for man and woman. It puts oneself in the drivers seat, and degrades sex to a purely physical act. But sex is not a merely physical act of love. It is a supernatural act where man and woman embrace in body and soul.