I'm of the opposite opinion. By this point in the story every fuckable girl wants a piece of MC. A Zoey that has been sleeping around would make her just another side girl in the waiting line for MC's cock.
On the other hand, a Zoey that hasn't been sleeping around brings a very dramatic contrast to MC's experience at the college. It would make MC realize that the two of them had a much stronger and meaningful bond that he realized, instantly elevating her to main LI potential.
Also, if Zoey has been sleeping with Bret, it makes Emma's jealousy and anger justified. It'd also give MC another reason to move on.
Lastly, it's not sex that makes Zoey connected to the San Diego characters. It's the friendship and feelings they've developed that connects them. It's obvious that Bret has feelings for Zoey. If he's going to come chasing after Zoey, he'll come, whether they have had sex or not.