I certainly don’t have the only possible answer, but my opinion is that it really depends on the aspect of the love one wants to emphasise. 性愛 is correct insofar as their love is sexual: they love each other in an erotic sense, and a desire for each other’s bodies is likely a part of their relationship. More generally, they share a sort of love that only manifests itself between people who are sexually attracted to each other. 恋 would probably still be accurate, but tends to focus on the heady sort of attraction that people in love feel. I think that the sexual dimension is less important when 恋 is used though, with romance and desiring closeness being more important. 知音 might be a good term describing who married couples (ideally) are to each other: each is the other’s closest friend, understanding everything about them. (And in Japanese, it’s another word for 恋人, so it works pretty well.)
EDIT: I see that you meant in English/Greek. My bad then, but in case you’re wondering what you might say in Japanese…
Depends on whom you ask. In Modern Japanese, 全然 is generally used with negative constructions and means ‘never’ or ‘not at all’. However, it used to be used with affirmative constructions to mean ‘completely’, with such usage being common as recently as just before WWII (if I remember correctly – there’s a Japan Times article about it… the title is ‘No Way Around No’, I think). I believe it’s still acceptable to use it that way, and I’ve definitely heard such a construction at least once in an anime. Negative constructions are still more common, but I don’t think a positive construction can be considered incorrect. The positive definition is still listed in dictionaries, after all.