How do you tell a straight friend you're gay?

So, how do you tell a straight friend you're gay? I'm a pretty "masculine - straight acting" guy. Yes, I hate that description but, it's accurate. I always need to tell everyone who mets me that I'm gay. They always awesome my girlfriend, or fiancé is female.So, my co worker wants to hang out this weekend, and doesn't know I'm gay. I don't feel comfortable telling him at my current place of work. I don't want him to think anything weird about it. I'm not trying to pursue anything besides hanging out. I just don't want it to be awkward.Thanks.