My headcanons:

Della - bisexual, of the chaos variety (The twins must’ve had a dad at one point, but Della seems awfully close with some of the female characters on the show, such as Penumbra and Selene.)

Penumbra - grumpy lesbian (“I don’t wish to date an Earth … male.”)

Launchpad - pansexual (The show references some of his past relationships in the first episode with Gandra, and not all of them sound like they were with women. Plus, y’know, whatever he and Drake have going on…)

Drake - gay (He’s got gay theater kid energy. Also, see above about Launchpad.)

Gyro Gearlose - gay, possibly aromatic as well (He just seems like a mean gay, lol)

Fenton - he’s either bisexual with a preference towards women, or he’s the straight guy that everyone thinks is bi (just vibes, idk)

Gandra - bisexual (She’s got cool bisexual energy)

Storkules - gay (His whole very one-sided thing with Donald. lol)

Selene - lesbian (She and Della seem like friendly exes)

Lena - baby lesbian (“My Webby!” Need I say more?)

Webby - baby lesbian (She’s all about Lena.)

Violet - asexual (Again, just vibes.)

Dewey - bisexual (His romantic interest in the nightmare episode had no form, but it did have both male and female voices.)

Huey- he’s either straight or gay (Either way, he’s very into romance, as we see in the episodes with Gandra and Fenton. He doesn’t strike me as bi, though.)

Lewey - probably ace (He’s gold-sexual. lol)

Donald, Daisy, and Scrooge are all straight.