In DOS2, you can romance any of the six companions and have a long well-built relationship that varies depending on how you treat them, how you feel about them and vice versa. The relationship does have an 'intimate' moment at the interlude between Act 3 and 4. You can also flirt with various NPC's along the way, but nothing permanent or long-lasting as your main companion romance of choice. Also, there is no restriction based on race and gender, wherein the previous BG games some companions were not interested in you if you weren't a certain race or gender.
According to sources in BG3, you can romance any of the companions according to Larian, but the nature of your relationship can vary. Apparently the Githyanki companion Lae'zel is from a culture where monogamy isn't a concept, so this might cause some friction if you decide to pursue a romance with her. Some companions might develop relationships all by themselves if they mix well together, similar in previous BG games. As to the issue of sexuality, I have no idea.
