Why do I feel uncomfortable talking to other guys besides my boyfriend? I don't mean talk as in flirting, like having a casual conversation with someone.

I don't know ... I talk too much to everyone too much :) so you have the opposite problem of me then :) anyway ... Guys are the easiest to talk to cos they're way more afraid of us then we are of them... Trust me on this one ;) I know stuff about stuff.

Probably your own insecurity.  Not sure how old you are .. But it's pretty common to feel uncomfortable talking to the opposite sex .. Especially for younger folks. 

You have whats called a guilty conscious for some unknown reason here so whenever you talk to another guy, you feel guilty like youre cheating on him. From past experience working with people and their relationships, this usually results from the person wanting to hide something like a lie or even maybe a secret affair and afraid of getting caught.  Most people in this situation feel like they've been caught and that cheater or guilty is written across their forehead.  This is where the truth will set you free so do some soul searching and remember a wrong in your past if applicable and time will eventually take care of this.  Good luck

This is classic insecurity talking . .. Especially when talking to the opposite sex. It can be very intimidating. Its very typical especially for someone who is very young.

How you interact with others is dictated by your personality and temperament type .. Not "guilt".