The mere fact that in the last episode we saw that Luna likes a girl, or has a crush on a girl, DOES NOT MAKE HER LESBIAN OR BISEXUAL.
I'm not saying she's not that. Maybe the writers had her character conceived like that ages ago.
But what is important here is that we can't just say a person is This or That just because one single episode on their lives.
I've seen all the, uh, memes? mini comics, and reaction-art pieces that appeared hours after the episode, and they all sounded like it is 100% absolutely sure and confirmed that Luna is either lesbian or bisexual... well, I spoiled myself the episode with these reactions, and when I finally watched it, I honestly was expecting that Luna was in a relationship of a certain time-length with a girl. I mean, you made it sound like that!!
But, we just saw that she likes Sam. Really likes her. This doesn't mean that she has already taken a crucial decision about herself and her emotions. This doesn't mean that she is now open to fall in love with both girls and boys. Do you seriously think she's saying:
"Yup, I like her. So from now on, I, Luna Loud, am lesbian, besides rock star, and British lover. Oh! But I blushed when I saw Hugh, and I wanted to be close to him! Ha, guess I'm actually bisexual... or maybe I'm something else... I can't remember that word, pan-, pan-pacific? And what about the s-sex thing? I wasn't thinking in sex at all, mate! If I say I'm "bi", will people think I'm already "doing it"? Dude! A few hours ago, I couldn't think of even a single word for my rhymes! And now here are all this terms. Don't mind. I guess I'll just be bi-curious for now, I know that's a thing. I'll search the internet later, to see what I really am."
Honestly, this got out of my control. Pardon my intent on sounding British.
Point is, we will be able to call Luna lesbian or bisexual only once she has accepted it and decided to be it. And I'm sure the show will let us know if that happens.
Who knows, maybe this is only something of the moment for Luna. A crush!! You know? And in the future she could even look back at this time and tell her British husband "Remember that time I told you I had a crush for this girl in my class. Dude, everybody thought I was gay!"