It can be understood from their literature, art (and ceramics), as well as the records of other countries that mentioned the Greeks.
The sexual history of ancient Greece is colourful and eye opening so lets look at sex in ancient greece.
Sex in ancient Greece was seen mostly from the eyes of men, as women were considered as commodities only.
Women generally were used for keeping a house (for the man to sleep only) and bearing his legitimate children.
The wives of Greek men were highly supervised, and could not move freely about, except if they were old and motherly (or grandmotherly).
Sex with wives was intended for children only, and thus performed in bed, and usually in some modified missionary position.
Fallatio was considered taboo (for a marriage), and as the sexual act was to beget children, the man wished to deposit his sperm as quickly as possible and be done with everything else.
The active form of sex in ancient Greece had the wives competing with "eromenoi" (adolescent boys) and "hetaras"(educated prostitutes) and even slaves in the home.
It appears that homosexuality (pederasty...
sex with young boys) was the regular sexual choice for the men.
It shows up in education, religion, art and politics.
Also Greek men were fond of prostitution, with all kinds available, from street girls to temple prostitutes (for very high fees).
Raping was also considered a norm, even in Greek religion, Zeus was seen to rape many women, such as Leda (a swan), Danae, Alkmene, and boys such as Ganymede.
Due to the high rate of homosexual relationships (always an older man with a young boy...
pederasty) non-conventional sex flourished (and still to today, as custom relates).
The older man would have his young lover (but the young boy would feel no love or desire for the older man) as his sexual object.
The Greeks believed that semen contained the source of knowledge, and thus depositing it in the younger man would endow him with wisdom.
The homosexual relationships of the Greeks were not strictly limited to men, as there were also lesbian relationships, and this too were pederasty in nature.
Greek society frowned upon same age sexual relations.
Young unmarried women could move freely, but if they did, they would be considered as prostitutes, and thus have to sell themselves,or be raped.
The Greeks had a few sexual taboos, incest being the first among them, yet it shows up in the Greek literature as being a fact of life in those times.
Fallatio was also considered a taboo, but widely practiced.
It was said that a man would normally have to beat his partner (young boy lover or prostitute) prior to the act (as a sort of Greek foreplay), to get the fallatio to be performed.
In part 2 fo this article, you can read about sexual positions in Ancient Greece and how many of them live on today as some of the most popular and effective positions that can be used in lovemaking.