One of the things the Harbinger came to represent was gratuitous sexuality. In a polar opposite to Garanos, who is very reserved and tight-lipped about such things, the Harbinger flaunts his lust (be it for power, or sexual in nature) and uses sexuality to manipulate people as he does to Styx here. By accusing Styx of being in love with Garanos, which isn't entirely untrue in the first place, he chips away at Styx's nerve to get Styx to present an opening to strike. The fact that Styx physically has Garanos in a compromising position already furthers the Harbinger's plan even more, by subtly suggesting that Garanos is already no more than an object.