Licit sex means sex within the license of a heterosexual marriage, both civil and religious, for the purpose of production and raising of God or Krishna conscious children.
Such sex is godly and divine as Sri Krishna expresses in the BGAII 7.11:
“I am the strength of the strong, devoid of passion and desire. I am sex life which is not contrary to religious principles, O lord of the Bhäratas [Arjuna].”
Other forms of sex are consequently demoniac and are to be abstained from for those humans who plan to elevate themselves, within the cycle of birth and death.
Animals naturally follow their instincts. And there is an animal body available for even the weirdest sexual expressions, which I shall not enumerate.
Everything can be used in Lord Krishna’s service, including the genitals and such engagement is clearly regulated.