A contraceptive pill, taken by the male partner of a heterosexual relationship, is currently a big goal of biochemical medicine. (Because it would make so much money.) Currently, most birth control techniques weigh more heavily on the female partner. A female-taken contraceptive pill definitely prevents pregnancy, but also causes some strange side effects. Weight gain, menstrual alteration, and water retention are common side effects. While some women take the pill for some of the beneficial effects alone (the pill has been known to clear up acne, and sometimes reduces the severity of menstruation), women who suffer side effects must either endure them or switch to condoms.
Hormone alteration has been the first angle into this. By altering hormonal balance, male fertility is deliberately impaired, until this is reversed by ending the medication. So far so good. This also tends to kill his sex drive. The hypothetical man's hypothetical girlfriend would not hypothetically be pleased.
So the next attempt was to mix estrogen, a "female hormone" with testosterone, a "male hormone." (Actually, both genders have both compounds, but in different ratios.) Hypothetically, the estrogen increase decreases fertility, and the testosterone reestablishes sex drive. (Testosterone causes sex drive in both genders.) However, the testosterone may also increase male fertility, in which case back to the drawing board.
A clever doctor, Dr. Christina Wang, at UCLA has discovered that a mix of progesterone, a "female" hormone, and testosterone, seems to be working the trick. So far, none of the usual side problems (loss of sex drive, gynecomastia, has been noted, and the worst side effect seems to be night sweats. However, testing could take another five years. After all, it would suck if there was some horrible side effect hidden in this, and suddenly millions of men are subject to this all of the sudden.
The female based pill works by simulating pregnancy. Since her body thinks she is pregnant, she does not ovulate, and if she has sex, there is no egg to be fertilized. The male pill seems to likewise stop sperm production, so there can be sex but not fertility. To the relief of heterosexual couples everywhere.
People should note that the male birth control pill does precisely nothing to stop STDs, and should not be relied on in sketchy situations. Just like the female pill.
I, as a man, look forward to it.