The mitzva of procreation is relevant even to Noaḥides (non-Jews), as God said to Noaḥ and his sons: “And you, be fruitful and multiply, abound on the earth and increase on it” (Bereishit 9:7). However, the specific obligations that require men to get married by the age of twenty (24 at the latest) and to have a son and a daughter (5:7-9 above) do not apply to Noaḥides (see Sanhedrin 59a).
If a Noaḥide had a son and a daughter and later converted to Judaism, he has fulfilled the mitzva of procreation, for even when he was a non-Jew, the mitzva was relevant to him (Yevamot 62a). There is a disagreement as to whether this is the case only if his children also converted, or if it is true even if they did not (5:4 above).
Since the mitzva for Noaḥides is a general one and is not an absolute obligation with specific parameters, a Noaḥide who has good reasons not to fulfill it – for example, if there is a reasonable concern that his children will be sick, or if there is a serious concern that he will not be able to educate his children to be decent people – he need not do so even le-khatḥila.
Some say that Noaḥides may not waste seed. We have seen (4:1) that this is one of the sins that was punished by the flood, as we read:
The earth became corrupt before God; the earth was filled with lawlessness. When God saw how corrupt the earth was, for all flesh had corrupted its ways on earth, God said to Noah, “I have decided to put an end to all flesh, for the earth is filled with lawlessness because of them: I am about to destroy them with the earth.” (Bereishit 6:11-13)
The Sages explain that the flood waters were as hot and thick as semen, for “with hot passion they sinned, and with hot water they were punished” (Sanhedrin 108b). The waters of the deep overflowed and drowned them (R. Ḥayim Palachi, Ḥayim Ve-shalom §16; Tzofnat Pa’ane’aḥ §30).
In contrast, some say that there is no prohibition for Noaḥides to masturbate and waste seed; since this prohibition hinges on the mitzva to procreate, which does not apply to Noaḥides, they are also not included in the prohibition to waste seed (based on Tosafot to Sanhedrin 59b, s.v. “ve-hu”; Igrot Moshe 4:116). It would seem, though, that even according to those who are lenient, such behavior is improper, for all desire should be directed toward increasing love within the framework of marriage.
. Sdei Ḥemed (Klalim, Ma’arekhet Zayin, klal 20) explains that according to Tosafot (Sanhedrin 59b) there is no prohibition for non-Jews to waste seed, because they are not obligated to procreate. She’iltot, which maintains that non-Jews are obligated, rules that they may not waste seed. However, Ramban and Rashba (above, ch. 4 n. 15) argue that there is no connection between the obligation to procreate and the prohibition against wasting seed. Thus, we can say that Noaḥides may not waste seed just as Jewish women may not masturbate – because desire must be preserved to increase love between husband and wife.
It is possible that the ruling in this case hinges on the underlying rationale of the prohibition. If the prohibition to waste seed stems from the prohibition of “Do not commit adultery” (following Or Zaru’a 1:124 and Smak §292), then perhaps Noaḥides are included, since they, too, are admonished not to commit adultery. If the prohibition to waste seed stems from the mitzva of procreation (following Tosafot), or from the prohibition of bal tashḥit (following R. Yaakov Ettlinger), then non-Jews are not included, even though wasting seed is still improper.