I don't know of the very earliest instances, and that would require some extremely difficult histeriography involving, say, dating the origins of various mythological accounts of homosexuality. But by the time of classical ancient Greece we have a lot of well-studied attestations of homosexual relations, and what is more, they are by then a relatively commonplace phenomenon. The definitive source of this is Martha Nussbaum's '[Platonic Love and Colorado Law]', a journal article expanding on her expert testimony on the extent to which homosexuality was accepted by Athenian elites in the classical period. It's fascinating stuff, and it is extremely thorough, running to 136 pages (!). What's especially important about Nussbaum's treatment is how it links the sympathetic portrayal of homosexual relations to the plight of contemporary homosexuals who may suffer from legal and social discrimination (the case she was serving as an expert witness in being a good example).