The Diary

That he was a virgin.

She doesn't write that Horace is a virgin. She writes that she is a virgin. As for Horace, it doesn't specifically say, but in the series it hints that he likes to party and do things of that nature, so it could be something along those lines, but that is just a guess.

But there is a diary contradiction in one of the final episodes, when she is IMing and her grandmother questions being so wrapped up in it. I forget he exact wording but I think grandma wishes for the days when teens wrote in diaries and the girl says something like nobody does that anymore.