Semen isn't the only thing Kyle sucks straight from Cartman's dick. A slew of feelings get funneled out through his urethra for the first time, this ugly thing that resembles affection, worse than he ever had it for Wendy or Butters. Apparently passive-aggressive Type-A personalities with no gag-reflex are his type.
Afterward, Kyle spits into the wastebasket and makes a face like he just sucked a lemon. He passes out diagonally across the mattress with all his clothes on, reeking of alcohol. Later that night, when he rolls over with the blanket wrapped around him, Cartman knows he's irreversibly fucked.