recently i went to a museum and there was a piece by toulouse-lautrec called “la goulue and her sister.” the description next to it said that although the women depicted claimed to be sisters, they were probably lovers. many lesbian couples pretended to be sisters to avoid scrutiny from society. this was in the late 19th century, so pretty recent compared to euripides’ work. it really is part of a broader cultural trend like op said, and it kind of surprises me that people don’t realize the lengths that lgbt people have had to go to in order to be able to express their true selves without putting themselves in danger.
the orestes and pylades thing plays into a broader cultural trend of veiling homoerotic relationships with the plausible deniability of them being related, giving an ‘excuse’ for them being close (see certain historians interpreting achilles and patroclus as cousins) which can be done both by the author, to evade the backlash a more direct portrayal might get, and by the community around it—especially with myths, which have been recontextualized, reinterpreted, and translated thousands of times. there is no one empirical canon that is true in mythology.
(and before anyone tries to ‘correct’ me here: yes, there were certain homoerotic relationships that were accepted and condoned in ancient greece, but that didn’t somehow translate to gayness being fine—it was only in the context of the erastes/eromenos relationship (which orestes and pylades did not fit, as pretty clearly evidenced by the amount of contemporary discussion about it), as a direct parallel to and imitation of the heterosexual dynamic, and as a prelude to marriage. intimate, loving, long-term relationships between adult men were in no way societally accepted or approved of, so yes, their relationship could absolutely have been treated this way by euripides or the people interpreting his work!)