future marriage still creepy and wrong.
You are judging the novel's premise without reading the novel.
It is handled very... logically. The couple were intimate, to save Spock's life
, in ST III. Spock didn't raise her as a daughter. (Their previous relationship had been more teacher/student.) Spock and Saavik then went many decades
avoiding the issue and both faced lonely, single lives, neither having met a new partner. Perhaps because
of the bonds formed during ST III.
Eventually, as two grown, long-lived, lone adult Vulcans, they decided to initiate a different relationship. They hadn't been teacher/student for decades.