What are the chances of giving up someone for someone you only met on a dating site?

I have joined a dating website a month ago and found someone i really like...his status though was 'ill tell you later' i find it a bit dubious and tried to not really pay attention with him... i sent him the first msg by the way as he looks great... eventually, it was followed by lots of messaging back and forth, then sms, then chatting, then exchanging a lot of photos, seeing each other on cam and i learned recently that he is currently attached, was married before too... both of us find each other attractive and like our personality.. basing on the way our chat goes... he seem to be really loyal to his gf as his main purpose was just to meet friends since he travels often to asia and would like someone to eat with and bar but nothing that far...the few weeks i have known him over the net, tells me that he is a nice guy.. so nice that my feelings grow as well as his to me.. but i know he will not leave her... what should i do? i like him a lot but i know i cannot have him :(