In this light, Billy's bond to Mandy may be more sexual than romantic. Or, at least, interlaced with his affection may be addiction to a sheerly sexual thrill that is heightened by her wanderlust. Of course, you might still expect the affection to wane if Mandy started consummating lots of her flirtations. Billy was aware of. Not that evolutionary psychology would rise or fall on the question of whether one man stayed in love with a truly promiscuous woman.
Individual people are very complicated and very different from one another.
On television, he remarked that this was a good metaphor for bringing Mandy to Temptation Island. Actually, I think it's a good metaphor for his entire attraction to Mandy. There's nothing unusual about Billy pining for Mandy. By the same token, there's nothing strange about Billy sticking with one temptress on Temptation Island rather than playing the field. If we take him at his word that he had a strong personal, not just sexual, attraction to her, then it's natural that he enter courtship mode.