I get what you're saying, but I still maintain that Sterling has voluntarily committed questionable acts. We can probably absolve him when he's affected by the recent, ultra-strength aphrodisiac. But for most of the story, we get to read enough of Sterling's thoughts to know that he knows better. He's been clear-headed enough to try and manipulate or deceive the women into sex several times, which shouldn't be hand-waved away with the drug excuse. I'm not complaining about it, if anything it's an interesting part of the story: to see if he can balance love and lust, or if he becomes a monster like Hunter.
Also, Sterling's lucky that Caspian was into being cucked. One day, in his pursuit of pussy, Sterling might cross paths with a jealous man who could murder him, not unlike some of the alternate endings where Hunter dies.