I read yaoi, not watch it but....

TBH I haven't really watched much yaoi, like I've watched all that I can find but if you're trying to open their mind about gay relationships yaoi anime probably isn't the way to do it bc they're usually not very true to life they're rather idealistic. I'd actually watch some live action thing like from USA but if you DO want yaoi I'd actually have them read some shonen ai bc that's wayyyy less idealistic sometimes (depending on which series you read) and there are a shit ton of good manga and manhwa shonen ai out there.