Why doesn’t he care that his friend fancies me?

QAll last year my partner was away studying Monday to Friday every week, and I took a job at a shop run by one of his mates. The mate kept hitting on me, I kept refusing. But each weekend, when my partner came home and I told my him what was happening, he just shrugged it off. What’s worse, he still goes drinking with the guy as if nothing has happened. I’ve left the job and have lost all faith in my partner – do I mean so little to him?