In principle, alcohol affects our judgment, so couples can have unprotected sexual intercourse, thus increasing the risk of developing sexually transmitted diseases or a long-term commitment they have not planned!
Women
Drinking alcohol more often than once or twice a week or drinking more than one or two drinks at a time can affect menstruation and fertility levels. Research has shown that drinking alcohol while attempting to conceive can affect a woman's chances of becoming pregnant and increases the risk of miscarriage.
For the first few weeks, a woman doesn't know if she's pregnant. By avoiding alcohol consumption while trying to conceive, she ensures that the fetus will not be exposed to alcohol and the possible harm this can cause during the early and critical period of fetal development.
Men
Drinking alcohol can affect sexual performance, since those who drink do not have full control of their bodies. Alcohol reduces the amount of sperm and consuming large amounts of alcohol can cause temporary impotence.
Drinking large amounts of alcohol can reduce testosterone levels in men as well as the quality and quantity of sperm. So men are advised to reduce consumption to one or two drinks a day if the couple is trying to conceive.
http://www.talkaboutalcohol.com/YoungPeople/FactZone/FactZoneFlash30.aspx