Are gentiles prohibited from performing Zera Levatala?
Some Poskim rule it is forbidden for even gentiles to perform Zera Levatala. Other Poskim rule that gentiles are not explicitly prohibited against performing Zera Levatala and it is thus not part of the seven Noahite laws. Others say that although the gentiles are not explicitly commanded against it, it became accepted upon even gentiles to avoid doing so and is considered like law, and thus we find that Er and Onen were punished with death for doing Zera Levatala.
 Rashba in name of Ramban, brought in Mishneh Limelech Melachim 10/7 “Venachzor”: “Although the gentiles are not commanded to have children they are nevertheless prohibited from destroying seed, as we see that they were all punished for doing so in the times of the Mabul.” See Mishneh Lemelech ibid who concludes “I do not know where this is hinted to in the Torah”
 Not listed in Rambam Melachim 10 as a prohibition; Tosafus Sanhedrin 59b “We know that Bnei Noach are not commanded in the Mitzvah of Peru Urevu being that whoever is commanded in Peru Urevu is commanded not to do Zera Levatala [and from the fact Zera Levatala is not listed as part of the seven Nohaite laws we see they are also not commanded to have children].”
 Shlah Vayeishev p. 170 “Go see how severe is the sin of Zera Levatala that Er and Onen were liable for death even prior to Matan Torah, and even though we do not find any prohibition for a Ben Noach. Nevertheless since they received the prohibition form their forefathers [therefore they were punished.”