With Drew and Jason, you have to remember that they've been living apart now, essentially since that summer. As far as he knows, his brother is still in that party life. Believe me, before the engagement announcement, there would have been some skepticism from the family about Drew having a serious girlfriend.
As I mentioned in my reply to Drew, Zach sees Corey as a bit of a shiny toy. He's had him for a while, so the newness was wearing off. There are also very few gay men in and around the city where they live now, so Zach hasn't seen Corey interact on a personal level with very many men. Corey and Drew love each other and are best friends, it rattled Zach to see someone he thought wanted his toy. Was it a shitty attitude to have? Yes. I know someone like Zach and he acted almost exactly like this when his girlfriend's male friend paid her any attention. Otherwise, he practically ignored her.
Thank you for your review and your support, Spikey