Due to some ancient laws of royal succession forgotten about (until the princess married two men at once to form a polyamorous relationship), when a person of royal blood has more than one consort, any sexual act where impregnation is possible must involve all partners equally contributing to the act of reproduction.

Martin and his habit of abiding by all the rules interprets this (with a bit of blushing) as double-penetration. Douglas and Theresa are both very interested in this idea, and rules are sometimes made to be followed. For the good of the sovereignty of course.