Not a male but I am sure plenty will pop in to reply later but just read your intro.
Why did he become colder at that point in time and yet still remained sexually excited to be with you? he didn't let it affect the sex side of the relationship though. Do you know who leaked your relationship to your coworkers, was it him? or another party? Have you found out where that work gossip started?
Something else was happening that he was hiding from you, there has to be a secret he held. Because it was not your fault that came out at work and would have affected you as much as him, so looks odd for him to blame his girl for it and stop talking to you, would he have been dating another woman at work as well and she told? He is being fishy about it, have you directly asked him ? Do you know any of his friends at work or otherwise that you could get answers from since he clammed up on you? Obviously you are desperate for answers and closure.
No his behavior is off. He has some secret you don't know. Does he think the work gossip didn't affect you too?
“The one you love and the one who loves you are never, ever the same person.”