Birth control is the practice of preventing birth from taking place. There are two ways this can be done - by preventing conception from happening or by destroying the foetus at some stage before it is born; i.e. abortion (gabbhapatana). Buddhism teaches that life begins at or shortly after conception and thus considers abortion to be a type of killing (M.I,265). To prevent conception from happening, either by using condoms, contraceptive pills, cervical devices or spermicides does not involve killing and is thus morally neutral. The ancient Indians practised douching to prevent conception and also made condoms out of animals' intestines.
That pretty much does it as far as sex and Dhamma is concerned. If there is any other aspects of the subject that you would like discussed please let me know and if I think I can say anything meaningful about it I will. From the 1st of next month I will discuss the positive emotions from the Buddhist perspective.