Basically, the male condom places a layer of latex rubber between the male and female to keep the sperm from getting to the egg. It is a contraceptive method.
In his chapter on contraception in Evangelical Ethics, John Jefferson Davis writes, "When consistently used, the condom can be an effective contraceptive device, having a pregnancy rate as low as three per 100 woman-years [this means that three in every one hundred women will get pregnant within one year of use]. It has the advantages of simplicity, low cost, and partial protection against venereal diseases. Its disadvantages include psychological distraction, dulling of sensation, and occasional breakage or leaking. There are no known health hazards associated with its use." (p. 34)
It seems like almost everything he writes is widely accepted. In my research, I have read some things (mostly published by the Couple to Couple League) suggesting that it's possible that, when there is long-term consistent use of condoms, and the female body is not used to her husband's semen at all, her body may end up having a harsher reaction to the semen when it is present. Basically, if you use condoms all the time and then stop using them because you want to get pregnant, you may experience problems for a period of time while the female body gets used to this "foreign substance".
Additionally, in The Art of Natural Family Planning, Kippley and Kippley cite a Washington Post article from 1989, which reads, "'Women who rely on birth control methods, such as condoms and diaphragms, that prevent semen from reaching the uterus are more than twice as likely to develop one of the most serious complications of pregnancy as are their counterparts who had been repeatedly exposed to sperm from the prospective father.'34 The complication is called preeclampsia or 'toxemia of pregnancy' and 'is the third-ranking cause of pregnancy-related death, following infection and hemmorhage.'" (p. 12)
Clearly, there are many virgins who enter marriage never having experienced the invasion of sperm in their bodies, their bodies don't end up putting up an all-out protest, and they get pregnant right away and maintain their pregnancies. However, these are things we should be aware of. It is one of the reasons that, while Chet and I were trying to prevent pregnancy for a short period of time after the birth of each of our boys, we used a combination of the fertility awareness method and condoms. When I was clearly infertile based on my charting, we didn't use anything, but during my more fertile times, we used condoms.
Basically, they line the vagina instead of having to be worn by the man. They are contraceptive in function. They can be inserted up to 8 hours in advance, so there's less interruption. However, they are not used very often because they tend to be more difficult to use than male condoms, they seem to have a higher failure rate (maybe because you have to learn how to insert them properly), they tend to cover the clitoris which obstructs female climax, and they are certainly much more expensive than male condoms. Some people appreciate the idea that the female can share in the wearing of the condom, but it seems clear that this is not enough to motivate large numbers of people to overcome the other obstacles related to using these devices.
Cervical Cap and Diaphragm
The cervical cap and diaphragm are both dome-shaped rubber devices inserted over the cervix to prevent sperm from entering the uterus. Both are generally used in combination with spermicides to optimize effectiveness.
The cervical cap is not widely used or recommended because of its many disadvantages and risks. I'm not going to spend a lot of time on it for these reasons, but if you're interested in learning more, please read an overview here.
The diaphragm, which is much more commonly recommended and used, is a dome-shaped barrier made of soft rubber that is inserted to cover the cervix in order to prevent sperm from entering the uterus. The woman must be fitted for the diaphragm, and because the shape of the cervix changes so dramatically throughout the cycle, it is important to be fitted at the time that she is most likely to be ovulating. The diaphragm is generally used in combination with a spermicidal agent. With a proper fitting and when used with a spermicide, the diaphragm is ineffective about 5% of the time (which means that 5 out of 100 women will get pregnant within one year of use). When improper fittings, improper use, or absence of spermicidal agents are factored in, the failure rate tends to be upwards of 20%.
Additional disadvantages include a greater risk of urinary tract infections and the need to be refitted and replace the diaphragm after pregnancy, surgery, weight changes, etc. Also, see the last three paragraphs at the end of the section on the male condom, as they apply here as well.
Many people enjoy the diaphragm because it can be put in place early in the day and left there so that there is no interruption in intimacy. Additionally, it seems that there is no noticeable diminishing in sensation for either the man or woman.
The diaphragm itself is contraceptive in nature, so it would appear that there is no inherent moral issue with its use as a rule. However, it tends to require the use of accompanying spermicidal agents to optimize its effectiveness....
Spermicides come in various forms, but they are chemicals that are used to kill the sperm cells before they can reach the egg to fertilize it. They are considered a barrier method, because they present an obstacle for the sperm to overcome on their quest to reach the egg.
In Evangelical Ethics, John Jefferson Davis writes,
Devices such as condoms and diaphragms are frequently used in conjunction with foams, creams, suppositories, and jellies, which contain spermicidal chemicals. Until the early 1980s there was little or no concern about possible harmful side effects of these chemicals. But then a study, conducted under the auspices of Dr. Herschel Jick and the Boston Collaborative Drug Surveillance Program raised the possibility of a link between the use of such vaginal spermicides and a higher than normal incidence of birth defects. Researchers studied the children born to a group of 4,772 women in Seattle, Washington during an 18-month period. The incidence of severe birth defects, including Down's syndrome, malignant brain tumors, and limb deformities, was twice the rate of those born to nonusers.35 While the total rate of serious defects was low--2.2 percent for babies of spermicide users versus 1 percent for nonusers--the results were still a matter for concern. Animal experiments have shown that the spermicidal chemicals can be rapidly absorbed through the vaginal wall into the bloodstream and then carried to the ovaries and uterus. Dr. Jick noted, "It would be prudent to give up spermicides at least two months before getting pregnant, and to stop using them immediately if you suspect you might have already conceived."36 (p. 35)
In The Art of Natural Family Planning, Kippley and Kippley write:
"The most serious problem with barrier methods is an increased risk of miscarriage if pregnancy occurs. 'Women who inadvertently become pregnant while using spermicidal contraceptives suffer about twice the rate of miscarriages in the first three months of pregnancy as other women, according to researchers at Temple University and the New Jersey School of Osteopathic Medicine.'30
Another serious problem: spermicidal foams and jellies may cause birth defects. This allegation was made in the medical literature in 1981, challenged, and reaffirmed by those who made it.31 In January 1985, 'U.S. District Courth Judge Marvin Shoob said Ortho Pharmaceutical Corp., which makes Ortho-Gynol Contraceptive Jelly, knew its product could cause birth defects and was negligent for not warning its users.' The court awarded a judgment of $5,100,000 to the parents of an unplanned pregnancy child born with birth defects.32" (p. 12)
Further links regarding spermicides:
"Birth Defects Tentatively Linked to Spermicide Use," New York Times, 1981
"Spermicide: Vaginal Route," Mayo Clinic. This article includes the following statement: "Many studies have shown that the use of vaginal spermicides does not increase the risk of birth defects or miscarriage."
In the end, there are quite a few studies that suggest that there is no link between spermicide use and birth defects or miscarriage. However, in my mind, it would seem reasonable, based on some of the research provided, to take the safer route and avoid spermicides if possible. I'm not a person who lives in great fear of things going wrong, so I don't think this is an opinion I've formed out of paranoia. Instead, I kind of see it like the Atkins diet... there are people who did lots of studies that showed that there was no link between heart disease and the Atkins diet. However, there is an uncanny number of people on the Atkins diet who develop heart disease and die. I'm not on the Atkins diet because I'm taking the safer route. It seems like I'd take risks to another person's life even more seriously.