One of the main problems in bisexual reproduction is how to get the sex cells together. Or - looked at another way - the whole business of sex is so pleasurable for human beings that the main problem is to keep the sex cells apart, i.e. to achieve effective contraception. In its earliest stages biological evolution managed to contrive a variety of means whereby the "male" and "female" components could come together for reproduction: the penis and vagina arrangement was "late" in the evolutionary scale. The earliest species lived in water and simply released their sex cells - the earliest equivalents of "sperm" and "egg" - to float about. Some of the cells had cilia, moving hairs, to propel them along.