Ok, firstly, I have to question exactly why how he manages his privacy in relation to online social networking sites bothers you so much. The fact that you choose to keep your accounts so rigorously separated, probably for all the right reasons given your profession (ie strangers who see you perform look you up), doesn't mean he is obliged (as your partner) to uphold the same standards. I understand that him not being as vigorous could potentially put you at risk because maximizing your privacy on Facebook requires your friends to have similarly stringent settings. But this doesn't necessarily mean you get to dictate how he manages his accounts, unless this was something you had negotiated when you started the relationship and/or added one another on Facebook.
You googling him isn't a huge deal, it would have been better to simply ask what you wanted to find out though, more forthcoming. I prefer that word to honest because it encapsulates these sort of situations where a person does something furtively. But, again, why does it matter? Why does him using the same username for all his online accounts bother you so much? Is it a) you feel your partners should feel/behave the same as you, b) you fear he may compromise your security or c) you feel this difference in online account security is a fundamental incompatibility.
If its closer to a, my response is that you need to be less controlling. If its b, you need to understand that not everyone's circumstances warrant the kind of management you require and perhaps you should mention it before you add people and ensure they are willing to adhere to your standards. And if it's c, well, not much I can say.
I wouldnt say you violated his privacy, but you weren't forthcoming about what you wanted to know and why you wanted to know it. That would be a red flag for me. If I felt this sort of thing was habitual, it would be a deal breaker. If I felt you thought my life had to mirror yours because we were in a relationship and now I was expected to make the same choices as you, I'd find that domineering and break up with you.