There are two contraceptive methods that are halakhically controversial. One is the use of a spermicidal foam, gel, or suppository that a woman inserts into her vagina before sexual relations. The second is the use of a diaphragm, a shallow, dome-shaped cup that a woman inserts in the vagina to block the opening of the cervix to block sperm from reaching the uterus. Spermicide is usually placed on the diaphragm in order to increase the effectiveness of the contraceptive. When a woman knows how to properly insert the diaphragm, the chances that she will conceive are very slim, but few women know how to insert it properly. In practice, among women who use either of these two contraceptive methods while regularly fulfilling the mitzva of ona, more than ten percent can expect to get pregnant over the course of a year.
Poskim who are lenient maintain that since the woman is the one who inserts the diaphragm or spermicide, and since the couple has sexual relations without any barriers between their bodies, it is not considered a waste of seed, only a means of preventing the sperm from reaching the uterus and fertilizing the ovum. Those who are stringent maintain that using spermicide is, by definition, destroying seed.
When there is a real need to avoid pregnancy, and for whatever reason birth control pills and an IUD are contraindicated, a couple may use these two methods. However, if spermicides and diaphragms were more effective, it would be permissible to rely on the lenient views only under extenuating circumstances. It is precisely because there is more than a ten percent chance that a woman using these methods will get pregnant within a year that they may be used if necessary (Ezrat Kohen §37). This is on condition that a couple using them have reconciled themselves that should a pregnancy occur they will accept it with good grace.
According to Rabbeinu Tam, this debate concerns the insertion of a mokh into the vagina after sexual intercourse, to absorb the semen and prevent pregnancy. However, all of these Tanna’im agree that it is forbidden to have sexual intercourse if the mokh was inserted beforehand, as the husband would waste his seed by ejaculating into the mokh. This is also the ruling of Responsa R. Akiva Eger §§71-72; Ḥatam Sofer, YD §172; Binyan Tziyon §137; and Rav Pe’alim YD 4:17.
In contrast, according to Yam Shel Shlomo (Yevamot 1:8), the disagreement in the Talmud concerns the insertion of a mokh before intercourse, and there is no problem of wasting seed because the couple has relations normally. The disagreement between R. Meir and the Sages is whether the three types of women must use a mokh or may use it. R. Meir believes a mokh is required, since pregnancy would be dangerous, but the Sages maintain that a mokh is not necessary since it is rare for these women to conceive naturally. This is the view of several Rishonim: Rid (Yevamot ad loc.), Rosh (Responsa Rosh 33:3), and by implication Ramban, Ra’ah, and Rashba’s student (Shita Mekubetzet, Ketubot 39a). This is also the ruling of Ḥemdat Shlomo, EH 46; Maharsham 1:58; Torat Ḥesed, EH 2:44; Ketav Sofer, EH 26; and Aḥiezer 1:23 and 3:24. Rav Kook is inclined this way as well (Ezrat Kohen §§34 and 37).
It would seem that R. Tam and those who agree with his view would forbid the use of a diaphragm or spermicide, just as they forbid use of a mokh. Nevertheless, many Aḥaronim who ruled leniently about diaphragms and spermicides, whose use has become common over the past century, reason that these forms of contraception are permitted even according to R. Tam’s view. R. Tam is strict about a mokh because the couple can feel it as a barrier between them, but these contraceptive methods allow for normal sexual relations, wherein the semen enters the woman’s body without the sensation of any barrier separating them (Maharsham 1:58; Maharash Engel 6:86; Si’aḥ Naḥum §94).
In any case, the mainstream position is the permissive one, which is the opinion of most Aḥaronim, and, in their interpretation, all Rishonim. Nevertheless, since some Aḥaronim prohibit the use of these contraceptive methods, it would have been proper to rule that they may be used only in extenuating circumstances, when there is a clear need to prevent pregnancy and the preferred methods of contraception are not options. However, in cases where the permit to prevent pregnancy is less apparent, it is proper to rule stringently and avoid these methods. But these contraceptives have a certain halakhic advantage over other methods, as they are less effective. Rav Kook writes that when the contraception is less reliable, it is possible that even those who are generally stringent would be lenient (Ezrat Kohen §37). Thus, permission can be granted more easily for spermicides and diaphragms, and a couple may use them if there is a real need to use these methods specifically. Of course, it is important for the couple to be aware that these methods of birth control are not fully effective, and they should be prepared to accept a pregnancy willingly should it occur. In such cases, contraception is like an “unintended” consequence (davar she-eino mitkaven), because it is neither absolute nor entirely desirable.