Yes, you can have sex in Baldur's Gate 3
One of the best things about Baldur's Gate 2 is the way the NPCs interact with each other. They're a mixed bunch, to put it mildly, and sometimes they won't get along, to the point that they'll occasionally fight or even refuse to be in the same party together. They'll take similar issue with your actions, too, if they start to think that you're not the leader they're looking for.
Larian's latest developer update promises more of the same in Baldur's Gate 3. The characters in the game will each have their own personalities, motivations, and goals, and under normal circumstances probably wouldn't have much to do with each other. But the presence of the Illithid tadpoles in their brains forces them to come together to survive.
Even so, conflict is probably inevitable, and Larian said that there will be "hundreds of permutations that determine where a relationship heads." That includes choices made during character creation: Like in Divinity: Original Sin 2, pre-made "Origin Characters" will have unique interaction options with NPCs, but custom characters will also have differing dialog options based on race and class.
Romancing NPCs will also be an option, which will play out primarily at the party's camp. This is where "the most intimate thoughts and feelings come into play," Larian said, as the party "lays down their weapons and talks at their most vulnerable, all through cinematic dialogs."
"We are trying to make these relationships feel real and feel like the relationships you have in the real world," lead writer Sarah Baylus explained. "You will meet some people who you have nothing in common with, some will be useful to work together, but that is going to be the extent of it. But there could be situations where you get on really well, you share similar goals and you love travelling together."
Larian also confirmed that, yes, you can have sex in the game—a lot of sex, if the storyboard that appears briefly in the video is anything to go by.
The Early Access release of Baldur's Gate 3 was delayed earlier today, but the good news is that it's only by a week: It's now expected to be out on October 6.