Is it as explicit as games like The Witcher III, or just a series of innuendos? And if there is anything explicit, is there an option to turn it off?
There was nothing explicit in D:OS2. Game didn't have cutscenes as such, so "sex scenes" worked similar to BG2 - mostly happened of screen, except both characters would wake in bed, possible with no clothing (aka. in underwear). Narrator did a bit describing, but again nothing too indulgent or naughty. There is some outside romance sex one can have but it's purely optional, and again - mostly humorous.
BG3 comes with higher production value, so it's anyone guess if they do special cutscenes for sex. Personally, I hope not, at least nothing explicit. Those rarely look believable and age really quickly. Less is more, I think. Whatever will Larian do, I think they will be more clever with it, then making a digital porno cutscene. I have to especially commend them for Red Prince's "romance scene", which was genuily hilarious and, dare I say it, hotter then any other sex seen in games that I had a displeasure of seeing.