OK, I get what you're saying, but let me ask you this: do you think when people say "no" they always mean "no"? Yes, you should respond, no always means no. But does it? I'm not talking ideal world, I'm talking real world. I'm pointing this out because I'm not trying to make a point that it should be OK, but that it happens.
In this situation, did it work out that way? Not really. I didn't really lay the whole thing out, but with both Zach and Brittany, it weighs on Gathan's mind. The whole scene with Brittany won't totally evolve until he meets with David. But if Brittany tells Kristin she liked it, it was most likely one of her first encounters with a guy and as she got into it, she felt guilty and freaked out.
If it was OK, then Gathan wouldn't even be thinking about it. If he did, he'd be talking with bravado, about how he totally rocked her world, and how she couldn't resist him. Instead, it tweaks at him every time he thinks about sex. He knows it's wrong. One of his victims (Zach) is seriously f**ked up, while the other seems no worse for the wear. If I was him, right now, I'd be guilty and very confused.