Well, that's objectively false, as the way they're treated most certainly creates distinctions (also, the fact that they can't have children who carry their characteristics with humans).
Being accepted by the Red Jennies in all likeliness influenced her inability to see (or want to see) much a distinction between elves and humans. The organization seems to be almost entirely meritocratic with elves and humans being allowed in with no hint of typical Thedosian anti-elf racism. Seeing poor elves and humans having to suffer from the same oppressive Orlesian nobles and chevaliers also probably put into her head that their basically the same and that any notion that elves are treated differently is, in her words, "stupid". Plus, there's the whole "I don't want to be seen as an elf because elfy elves like nature, halla, history, scary elf ruins, making elf babies, magic and stories about how humans took everything from them. I don't fit that sort of culture, but if elfy elves are right and that's what being elven is all about than what does make me? Less of an elf? A flat ear? A wrong thing? A nothing? Well, that makes me feel like crap so how about I just be a non-racial, poor person that fights against nobles that try to hurt the innocent. Yeah, that's why better."
And for the reproduction issues. Well, Sera is a lesbian so that's kind of a non-issue for her personally, and judging from a banter with Solas she hates the idea that elves must mate with elves to ensure the continuation of the race . I mean, at best Sera might ended up adopting a child, which based on her desire to have a conventional wedding and marriage, and her talk of passing her baking skills down one day, that might be a thing that she'll actually do at some point.
Then the question becomes "how can she be led to see it?" Maybe just explaining things the way I've explained them here would help?
This isn't exactly the most usefully answer, but I'd imagine that giving her time to figure it by herself would be the most effective method. Sera's that person you knew years ago who acted like an overly defensive arsehole that didn't take criticism all that well, but years later managed to grow up a little, get over themselves and reconsider earlier prejudices.
I mean, that's basically what happens in Trespasser.