If the Inquisitor truly loved Sera, she would be prepared to give up on her "elfiness".
Cassandra ends up loving the Inquisitor, even if he does not believe in Andraste, even if he is an elf, even though he's a mage. Three things she normally would have looked at and judged someone for. So, no.
Sera is being immature.
A real relationship, one that's adult, and mature. Is about two people coming together and they don't necessarily have to agree with everything to love each other. That's how relationships work.
Fintan doesn't believe in the Maker, but respects Cassandra for his beliefs. And even with a bit more freedom, he supports her. He may not agree with her on every time, but he finds her passion, her aspiration something to admire to. He'd never ask her to change. Just as much Cassandra has never asked him to change. She becomes acceptant that there will be things they disagree on.
And if we expand this in real life.
That's how real relationships work. I have dated Christian woman before, I am not Christian, nor religious, I'm Aethist. But I never once had someone tell me, that now that we have been dating for IDK 2 months I must give myself to Jesus Christ or else the relationship is over.
A healthy mature relationship is one of disagreements, and communication.
As stated earlier, we need to stop trying to achieve the ideal fairy tale romance. Because it doesn't exist. A relationship is hardwork, but is also love. You will hate each other, disagree, from time to time. But you also love each other, and agree. You need communication and responsibility, but there's no magical perfection in a relationship.
If you love someone you work towards loving them. But that doesn't mean shedding your values or worth for someone.