One, there isn't any exclusivity talk. Two, you have only just switched to a relationship after fucking around with who knows how many girls. It'd be fairly normal for Fuckface to slip for a moment here and there, especially if heavily drunk. Three, the game can deck you even for things like mild flirting, without touching any asses at all.
There's no real human beings in existence who are immediately cut off any and all sorts of potential flirting on the grounds they have a bf/gf. Hell, if the game applied these "rules" equally the Sage would never return Fuckface's "I love you" because she's flirted enough with Josy in the last couple episodes to get her "cheat counter" past the cutoff point.