Yeah, it's a little more difficult with other SHs, as most of them aside from Sanji and arguably Brook plus Chopper don't really show romantic interests (in Brook's case it's more like a running gag, and Chopper was shown infatuated by that deer mink), but it's easy to imagine them either straight or bi. Nami though really gives off that les vibe, the most chemistry she had with anyone was Vivi, and her "happiness punch" was anything but seduction.
Ahaha, as for Usopp, I actually remembered that Oda mentioned in some SBS that it was Usopp's influence to get Luffy interested in girls (vol 54 or something). So yeah, Usopp def feels like someone open for any kind of love, although not actively pursuing it unlike Sanji. With Sanji being a mess, I dunno, I feel like he needs to get laid first to start thinking rationally, so far he's hell-bent on his SO being a cute girl (although he did hit on Kiku, so a cute trans girl is most likely also an option for him). Zoro and especially Luffy are totally asexual, with Luffy we even had a confirmation on the Amazon Lily, where he was immune to Boa's charms, and I can easily imagine Zoro there with the same outcome. Zoro gets somewhat flustered by Tashigi, but that's because she reminds him of Kuina, nothing else.