I think the romances in The Witcher are probably more "mature" than basically any other game.
Hear me out.
You have two major types: The quick fling and the commited relationship. The former is Geralt banging anyone with two legs and a vagina. The latter though is what is particularly "mature"
Geralt can enter a relationship with either Triss or Shani. While not made explicit at the time, Geralt is remembering his true love, Yennefer, and is confused. Both women understand this. And neither relationship really kicks into gear until there is a child they need to take care of. Thus, they both "settle" for the sake of raising and protecting a child.
In TW2, Geralt and Triss have a much more casual relationship, but it is clear during dialogue and cutscenes that Triss is conflicted. She wants to be with Geralt, but she also understands that Geralt still cares deeply for Yennefer. And the more he uncovers of his past, the more he'll uncover of her.
Whereas every Bioware relationship since the BGs has basically been the "quick fling", just spread out over a more realistic timeframe. You sit and talk to your companion a few times, flirt a bit, and then you boink. TW(1 and 2) just take that and shrink it down to a few hours.
That's not to say that either is particularly deep or "meaningful". But it is nice to have a more "depressingly realistic" example, as it were.