Boba Fett telling Leia that rape and premarital sex are immoral makes even more sense in light of the retconning of his background and Mandalorian culture. The latter places a heavy emphasis on marriage, family, children, and staying battle-ready at all times. To rape someone means you might father a child that you neglect to raise as Mando'a, making you da'buir (no longer a father), which is an unforgivable sin to them.
Even more poignantly: his wife was raped by his commanding officer, and he killed the man in revenge. Boba Fett was also completely faithful to her, and so far as we know was never with any other woman, further explaining his repugnance for either of those ideas (this conflicts with earlier stories, which state he's never been with a woman, period).