Plato, Laws 636c

ὅσαι τῶν ἄλλων μάλιστα ἅπτονται τῶν γυμνασίων· καὶ εἴτε παίζοντα εἴτε σπουδάζοντα ἐννοεῖν δεῖ τὰ τοιαῦτα, ἐννοητέον ὅτι τῇ θηλείᾳ καὶ τῇ τῶν ἀρρένων φύσει εἰς κοινωνίαν ἰούσῃ τῆς γεννήσεως ἡ περὶ ταῦτα ἡδονὴ κατὰ φύσιν ἀποδεδόσθαι δοκεῖ, ἀρρένων δὲ πρὸς ἄρρενας ἢ θηλειῶν πρὸς θηλείας παρὰ φύσιν καὶ τῶν πρώτων τὸ τόλμημʼ εἶναι διʼ ἀκράτειαν ἡδονῆς. πάντες δὲ δὴ Κρητῶν τὸν περὶ Γανυμήδη μῦθον

that especially encourage the use of gymnasia. And whether one makes the observation in earnest or in jest, one certainly should not fail to observe that when male unites with female for procreation the pleasure experienced is held to be due to nature, but contrary to nature when male mates with male or female with female, and that those first guilty of such enormities were impelled by their slavery to pleasure. And we all accuse the Cretans of concocting the story about Ganymede.