Why do I keep fantasizing about a guy I've only had a five minute conversation with? ?

The terrible thing about this is that the guy is in the same band with my current boyfriend. I don't know anything about him I just know that him playing the drums is the hottest thing I've ever seen! I am so ually attracted to this guy, but I'm really in love my boyfriend. I don't want to be unfaithful to him. How could somebody be so infatuated with someone that they barely know but still be so in love with their significant other? I just need to know if anyone else has been in this situation or has any advice. The devil on my shoulder is whispering in my ear saying do it..