A serpentine alien travels from one solar system to another to spread his gospel of universal love. He is assisted by a First Mate who doubts the efficacy of his captain's program. They land in a desert on a planet where every species can fertilize any other. As random matings would result mostly in nonviable offspring, species have adapted by making insemination extremely difficult with any mates but those that a female deems acceptable. A humanoid female comes to the desert to mate with her husband, thus avoiding any chance of a random seed entering her guarded womb. She is mesmerized by the spaceship captain, and lets her defenses down one by one, allowing his seed to enter (there is, however, no physical penetration of bodies). Aided by a computer, the First Mate projects that her son will, as planned, preach universal love -- but that this will lead to the end of the humanoids as a distinct species. He also takes the opportunity to remind his captain how badly things went on Earth when he inseminated a woman there; her son also preached universal love, but with destructive results as the message was not universally received.
As to publication, the story was in English and almost certainly in an anthology, but as to when, I can't remember. Most likely read in the 1960s to 80s.