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 later in Legends. In the Republic Commando Series (some of which was used in the still-canon Star Wars: The Clone Wars), 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. Furthermore, he's married, and while he and his wife are estranged, marriage is still Serious Business.
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 bar one occasion as far as we know was never with any other woman, further explaining his repugnance for either of those ideas (this conflicts with the story here, which says he's never been with a woman, period).
It would also explain why he added Drugs Are Bad to his rant; an addict is in no condition to fight, and spice dealers can endanger a whole community by undermining their battle-readiness and cohesion.
