Plus when Rebecca herself did that (when she ran for Miss Douche), everyone was kind of horrified.
Well, and especially under the specific circumstances - Rebecca stalked Josh and then he left her at the altar, after which he apparently believed that she was going to murder his mother. Nathaniel spent months negging Rebecca, then when they got together, it was costumes and sex games and then she spent months as his side piece, after which she pushed someone off a building at his party. SHE SLEPT WITH GREG'S DAD. You don't come back from that, I don't care how much of a cartoon the show is supposed to be.
Of the three, Nathaniel has the least weird baggage, but to believe that Rebecca and Nathaniel end up together after their shared history still stretches belief for this viewer.