hey.. i agree with you 
you see most people wont exactly jump from girl to girl like
nothing. usually it only starts when their relationship has
problems.. the thing once inpenetrable now becomes something like a
blown up balloon that can pop anytime. and that's when you come
into the picture.
there are some questions.. are you very nice to him? are you always
around when his gf isn? do you always care for him and do lotsa
things for him?
if there's a yes, MAYBE his girlfren isn doing this things, and
then you are like always there to do those things that a girlfriend
should do, SMSing him, going out with him etc etc.. so he will
naturally feel something.. but it might or might not be love, it
really depends. it si like you're filling up the space within him,
the emptiness that he felt when he was with his girlfriend, and he
then will start to reconsider the relationship with his girlfren,
because you appeared.
it's normal for you to do wat you did, felt wat you felt, it's
really wat most pple will do.. u like him rite.. i think this kind
of things happen to many pple b4, me included 
but.. wat decision you make will
determine alot of things..
see if u decide to be with him, when the day his girlfren comes
back, will he have second thought bout u, or is he really in love
with you?
if u decide not to be with him, would it be an opportunity lost? a
door that's wide open in front of you but u did not choose to
enter? there are many otucomes, many consequences, many situations,
it all interlinks.. so think and chose wisely girl
by the way, try thinking about the 'jiu cheng kao' the part when ke
something [the very fierce with hubby one] and her husband quarrel
alot, and then the biao mei very good to her hubby? yah.. mayeb u
can get some light from there 
