Buffy had already taken their "relationship" down that road. Spike had been encouraged by the two previous kisses. He thought they meant something emotional. She denied it. He was pissed off and initiated a fight with her, she took advantage of the anger, frustration and violence to initiate a physical relationship that was angry, frustrating and violent.
That was what she wanted with him, and since it was all he was going to get, it's what he gave her.
His comment was 100% Spike, and the way she took it was as a confirmation to what she had already decided was going to be the tone of their relationship.
Had she wanted things to be friendly and playful, she could easily have interpreted the comment differently, and would have. People make "crude" comments to their lovers in the heat of the moment, or afterglow, and it's part of the fun. A vampire saying that to his Slayer lover is a compliment, not an insult, and he clearly didn't mean it as an insult.
Buffy set the parameters of the relationship. She initiated every previous kiss, and jumped his bones in the crumbling house. An offhand comment by him the morning after wasn't going to change everything.