It wasn't that Ron wasn't annoyed by the snogging joke, but since it had come from someone in Hermione's family, things were slightly different. Had it been George, he would have just shot a hex at him and be done with it. This wasn't the same. These people weren't his family. He couldn't act like the stubborn little git he was when he was being mollycoddled at the Burrow.
"That's for your parents," he said about the basket she was taking. "I've got something for you, but..." Well, he wasn't going to give her his gift with all those people around. It was sort of personal, and Ron liked his privacy when he could have it. The P word was practically nonexistent in his house, which was probably why Ron withheld a lot of his emotions. They were too easily made fun of.
Ron followed Hermione into the house, only briefly looking around and suddenly having this Kenneth bloke by his side. He just grin awkwardly at everyone he made eye contact with. All the while, he was trying not to make judgment calls about these Muggles. He was sure that if they came to his house, they'd be just as shocked as he was.