Forgive my short and incomplete comments. The reference is to the Symposium. Without time to give context, the important part is as follows (however, context is actually not too important—the point is that monogamous committed homosexual relationships did exist, are not just a construction of our time).

But the offspring of the heavenly Aphrodite is derived from a mother in whose birth the female has no part,-she is from the male only; this is that love which is of youths, and the goddess being older, there is nothing of wantonness in her. Those who are inspired by this love turn to the male, and delight in him who is the more valiant and intelligent nature; any one may recognise the pure enthusiasts in the very character of their attachments. For they love not boys, but intelligent, beings whose reason is beginning to be developed, much about the time at which their beards begin to grow. And in choosing young men to be their companions, they mean to be faithful to them, and pass their whole life in company with them, not to take them in their inexperience, and deceive them, and play the fool with them, or run away from one to another of them. But the love of young boys should be forbidden by law, because their future is uncertain; they may turn out good or bad, either in body or soul, and much noble enthusiasm may be thrown away upon them; in this matter the good are a law to themselves, and the coarser sort of lovers ought to be restrained by force; as we restrain or attempt to restrain them from fixing their affections on women of free birth. These are the persons who bring a reproach on love; and some have been led to deny the lawfulness of such attachments because they see the impropriety and evil of them; for surely nothing that is decorously and lawfully done can justly be censured.