I'm assuming it might be related to the guilt trip that Jamie sends Brittani on once he catches the aftermath of her and Guy. That along with the fact that she might feel 'taken advantage of' due to the state she was in prior to the titjob, even if that's not necessarily what happens since she's the one who suggested it. That's my take at least. She seems a bit more laid-back with Guy if he's not in a relationship with her (so far) and if he's been nice and respectful towards her.
Yeah, this. As you'll see when you meet her later at the Chens' store, and then in the first chapter of S2, she's more comfortable with Guy if he passed up her offer without explicitly shooting her down for a deal-breaker reason (age, girlfriend, etc.)
I try to manage the relationships in ways the player might not always expect. While the standard wisdom in AVNs is basically fuck girls >> max points >> fuck girls more, I want to offer different ways to get there. Just because Britt is left a little cold by that encounter doesn't mean she hates the player. She's just feeling shitty about herself. What the player does in following interactions will determine the shape of their relationship.