Oh, I agree. There is no reason to depict premarital sex between our elvish characters. And while we will be implying a sexual relationship post-wedding, we are hardly going to include explicit sex scenes. I imagine we'll have some romance and some kissing, but all very tame PG kinda stuff. I would not expect any of our depictions of sexuality to push the envelope on a TV-14 rating. I think that by avoiding any tendency to show them as 'repressed' or 'legalistic' about this, we'll avoid making it seem weird/stilted.
We show the courtship of Finwë and Míriel, and later of Fëanor and Nerdanel. This will set the standard for what the audience will expect from us when it comes to elvish relationships. We'll want to establish when something deviates from the norm, so we'll likely show that there is something off about Eöl and Aredhel. I'm not sure exactly how we'll plan to portray 'not wholly unwilling' but it won't be entirely innocent. Our attempts to show that Finwë's remarriage to Indis is shocking and taboo is already tough enough - it's really blasé to the audience that a widower remarries after his wife's death. Portraying that divorce is 'impossible' for elves, even if your spouse is dead, is going to be a bit of a tough sell. Still, it helps make their culture different!
I am merely pointing out that the audience will be expecting Beren and Lúthien to be, for lack of another term, sexually active. It's okay to disappoint their expectations, but it's good to be aware of them, too. The idea of a couple declaring their love for one another, but then actually waiting for the family's blessing to get married and actually get together is going to feel weird/foreign/old-fashioned to the audience. That's okay, though. The reason to keep this in mind is because when you portray someone as overly 'innocent' like this, the audience becomes skeptical and thinks that, in reality, the characters are behaving shamefully in some way - that that's not what's really happening. There is more of a set-up for watching someone go back on their word, break their vow, cheat on the fiance they aren't sleeping with, etc. I don't mind if the audience is disappointed when there is no hidden agenda, but again, you have to be aware when you step away from their expectations.
Whether or not we want to go from 'no divorce' and 'no premarital sex' to 'no homosexuality' is another question.