It was sometime in my 20's that I figured out I wasn't straight...
I even tried to erase/get rid of my 'gay' thoughts/feelings one spring.
It was not until a few years ago that I truly admitted/accepted (to myself) that I was bi, and more towards the gay end of it.
I've never truly acted those feelings (watching gay porn/jacking off to the thought of guys/etc doesn't really count LOL).
I've never dated/had a girlfriend/etc either. (not really interested, maybe i haven't met the right one, maybe i'm too shy, or maybe too 'gay'.. IDK )
Anyway in your case I say "don't cheat". Only hurt can come from it
One of my personal beliefs is that no-matter if its girlfriend/wife/boyfriend/husband is that you should stay truthfull to them (I would also expect to be treated the same...)