A brief overview.
Aphrodite and Eros were key deities of love and sexual delight, both gifting mortals and gods alike with desire as well as cursing them with burning passion. The ancient myths are filled with tales of nymphs and mortals being ravished by the gods and more beastly creatures such as Satyrs. (The subject of rape is a larger topic that I might address at some point.)
Sex and sexuality in Ancient Greece were more liberal than today in some ways and more strict in others. A male citizen could partake of a multitude of sexual acts, with both males and females, though female citizens were more restricted.
Women were the guardians of citizenship. An Athenian citizen had to make sure all his wife's children were his. To keep her away from temptation, she was locked away in the women's quarter and accompanied by a male when she went outside. If she were caught with another man in flagrante delicto, the man could be killed or brought to court. When the woman married she was a piece of property transferred from her father (or other male guardian) to her husband. In Sparta, the need for Spartan citizens was strong, but women were encouraged to bear children to a citizen who would sire well if her own husband proved inadequate. There she wasn't so much her spouse's property as the state's -- as were her children and her husband.
Prostitutes were despised then as they are today, although for slightly different reasons. They might have been looked upon as victims (of pimps), but they were also greedy and deceitful. Even if they were honest financially, they used makeup and other artifices to make themselves more attractive.
Sex between wife and husband was just one of many choices available -- at least to the male. There were slaves of both sexes, concubines, and hetairai, all of whom were available, if only for a fee. Men could also try to entice a young man just past puberty. These relationships were the ones celebrated on vases and in much of Athenian literature.
In Plato's Symposium (a treatise on Athenian eroticism) Aristophanes offers a colorful explanation for why all these sexual options existed. In the beginning there were three types of double-headed humans, varying according to sex: male/male, female/female, and male/female. Zeus, angered at the humans, punished them by splitting them in half. From then on, each half has forever sought out his other half.
Homosexuality was also more common, especially in Sparta where homosexual relationships were even encouraged to help form comradeship between soldiers. Pederasty was also practises, where a more middle aged man cultivated a relationship with a teenage boy in order to teach him the ways of men, usually philosophy, sex and war.