I read in the paper the other day that a persons liklihood of fidelity is largely determined by genetics. I believe that. Not to remove personal responsibility, but guys like this have a natural urge to stray after a while. Not necessarily because of low-self esteem, or shabby morals. It's the way they're wired.
Anyway official get your girlfriend to take a take-home pregnancy test. What's the point sitting around sweating bullets for the next 2-3 weeks. In all liklihood your girlfriend is not pregnant. It's possible of course, but probably you'll get away with it this time.
Is it burnin'? Well, f-ck, now you're learnin'.