(7) But Er, Judah’s first-born, was displeasing to GOD, and GOD took his life. (8) Then Judah said to Onan, “Join with your brother’s wife and do your duty by her as a brother-in-law, and provide offspring for your brother.” (9) But Onan, knowing that the offspring would not count as his, let [the semen] go to waste whenever he joined with his brother’s wife, so as not to provide offspring for his brother. (10) What he did was displeasing to GOD, who took his life also.

ושחת ארצה AND HE DESTROYED ONTO THE GROUND — He 'threshed inside and winnowed outside' (Genesis Rabbah 85:5).

Rav Naḥman bar Yitzḥak said: As it is written with regard to Onan: “And He slew him also” (Genesis 38:10). This indicates that he, too, died the same death for performing the same transgression as his brother. The Gemara asks: Granted, Onan engaged in anal intercourse because he did not want Tamar to give birth as “he knew that the seed would not be his” (Genesis 38:9). However, with regard to Er, what is the reason he acted in this way? The Gemara responds: He did so in order that she not become pregnant and become less beautiful as a result of her pregnancy.

As Rabbi Yoḥanan says: Anyone who emits semen for naught is liable to receive the punishment of death at the hand of Heaven, as it is stated with regard to Onan, son of Judah: “And it came to pass, when he engaged in intercourse with his brother’s wife, that he spilled it on the ground, lest he should give seed to his brother. And the thing that he did was evil in the eyes of the Lord, and He slew him also” (Genesis 38:9–10).

s.v. "And behold, Fructification and Muliplication": ...And possibly this is also an abide-and-refrain, because He who commanded Fructification and Multiplication also commanded not to waste semen.

Rabbeinu Tam says…and the woman…is not cautioned against hashchatat zera, since she is not commanded in pirya ve-rivya.

It was taught in a baraita: The Generation of the Flood all were spillers of zera in vain. There were among them astrologers, they said, the world will last no less than six thousand years, we will not procreate, and we will live for as long as the world lasts. God said to them: You have made yourselves paramount [ikar]; behold, I will uproot [oker] your legacy so that you are not counted in the reckoning of the world. Whence are these matters known? For it is written regarding Onan, “so that when he had relations with his brother’s wife he would waste to the ground [shichet artza],” for he would get himself heated and spill zera in vain, and it is written regarding the generation of the flood “for all flesh ruined [hishchit] its way upon the Earth [al ha-aretz].”

…For even though they [women] are not commanded in pirya ve-rivya [procreating] and the Torah permits desisting from it, it is a prohibition of hashchata [of zera]…regarding hashchata “all flesh” is written.

The Sages taught in a baraita: During the first three months of pregnancy, sexual intercourse is difficult and harmful for the woman and is also difficult for the offspring. During the middle three months, intercourse is difficult for the woman but is beneficial for the offspring. During the last three months, sexual intercourse is beneficial for the woman and beneficial for the offspring

…a woman who does not go through puberty, that they were not prohibited in sexual relations on account of not being fertile…

You asked about a woman who has a blockage in her reproductive organs, so that the male member does not thresh as is fitting for it and because of the blockage sometimes he threshes externally, and he always scatters [ejaculates] externally. And it is doubtful to you if we say that since he intends to have intercourse as is fitting, it is permissible as it is to have intercourse with an infertile woman…It seems that it is prohibited. Since he always scatters externally, we call it “wasting toward the ground”…

Hashchatat zera is not on account of causing the zera to go to waste without conception, but rather the prohibition is on account of casting his zera in a place where it was not permitted to cast it, for the Creator decreed that a male may only cast his zera inside a female, who is of his species, in the place that is designated for casting seed.

A woman fills that place [the vaginal canal] with wadding to absorb the zera so that she not become pregnant…

Rav Beivai taught a baraitabefore Rav Naḥman: Three women may engage in relations with a contraceptive resorbent, a soft fabric placed at the entrance to their wombs to prevent conception, despite the fact that this practice is generally prohibited. They are as follows: A minor, a woman who is already pregnant, and a nursing woman. The baraita specifies the reason for each exception: A minor may do so lest she become pregnant and perhaps die; a pregnant woman, lest she be impregnated a second time and her previous fetus becomes deformed into the shape of a sandal fish by being squashed by the pressure of the second fetus. As for a nursing woman, she does so lest she become pregnant and her milk dry up, in which case she will wean her son too early, thereby endangering him, and he will die...This is the statement of Rabbi Meir. Since it is assumed that a minor who is less than eleven years old cannot become pregnant, she is considered to be in no danger. And the Rabbis say: Both this one and that one, i.e., in all these cases, she may go ahead and engage in relations in her usual manner, and Heaven will have mercy upon her and prevent any mishap, since it is stated: “The Lord preserves the simple”

They have intercourse with a moch – They are permitted to insert a moch [cotton wadding] in the place of intercourse [vaginally] when they have intercourse in order that they not become pregnant.

Rashi explained that it is permissible to have intercourse with a moch, but other women are prohibited because of hashchatat zera…

…Rabbeinu Tam says that before intercourse it is certainly prohibited to insert a moch there, for this is not the way of intercourse, and it is like casting zera on trees and on rocks when he ejaculates onto a moch, but if she inserts the moch after intercourse, it does not seem that it should be prohibited for the man has intercourse as is his [usual] way…and “they have intercourse with a moch” that is taught here means they [the three women] need to use a moch.

Have intercourse with a moch. Rashi commented “They are permitted to insert a moch [cotton wadding] in the place of intercourse [vaginally] when they have intercourse in order that they not become pregnant, and [other early authorities] raised a difficulty regarding [his position]…and therefore explained they [the three women according to Rabbi Meir] are obligated to have intercourse with a moch because of risk to her life or the fetus’s.

...For I have heard that nowadays doctors give women a sort of thin rubber cover, which is called in their language a pessary, with which they cover the cervix, which is the uterus, all around, so that it does not take in the zera. And if so, intercourse is as with other women—but just as with a pregnant woman the cervix is closed, so too with this [device]. And it is not similar at all to having intercourse with a moch, where the man is like one who casts his zera upon the moch, which is not the case in our situation, and one can say that everyone would agree to be lenient in a situation of risk to life.

What emerges in my humble opinion is that a moch at the time of intercourse is not the regular manner of intercourse but like one casting his zera upon wood and stones, or like the act of Er and Onan, in the language of the early authorities. And this is not solely a matter of the fate of the zera but that it is impossible to thresh and to sow [zera] as usual, as in Rashi’s description she fills that place [the vaginal canal] with mochin. But it is clear-cut that the diaphragm is not similar to this, and in Maharsham 1:58 he wrote this based on logic, and so was the opinion of my grandfather, who said that the diaphragm is not like casting zera upon wood and stones…