How can you tell if you're dating a gay/bi guy?

I have a consistent fear that every guy I date is secretly gay. If a guy doesn't go down on me, or doesn't always try to have sex, I assume he is gay. I'm seeing a guy who I'm crazy about (we work together and we're both 20), but he is very romantic and respectful, which I love, but it worries me because I'm used to guys who act like cavemen. (I prefer the way he acts, but I'm just worried that it's too good to be true). He doesn't pay a ton of attention to my boobs, but whenever we make out, even if it's just kissing, he has a boner. He also is very respectful of gay guys even though they make him uncomfortable (a good sign, as many gay bashers are really gay). He also doesn't believe that a person can be bisexual. How do I stop worrying about this and just live life? Everyone tells me he's straight, and as far as I know, he seems straight. I'm just sooo paranoid!