Sephiroth was not wrong - Genesis did want this. Judging by the intensity with which he begins to kiss back, he has been holding this desire back for a long time. (All those times their eyes would lock while sparring, all the glances, they were real, they did mean something.)

Sephiroth may not know what to do next, but the fervor with which Genesis kisses into him, pushes his tongue into Sephiroth's mouth, nips at his lower lip, it brooks no hesitation. Sephiroth shuts his eyes and allows Genesis to take the lead. His heart beats faster, his pulse thrumming in his ears. It's desperate, and raw, and yearning, as if the two of them were wild teenage rookies barely out of basic, and not two of the most powerful men on the planet.

Sephiroth breaks apart from Genesis and whispers, gasps, "I always wanted this, Genesis. I always wanted you."