Two things.
First, if you are both married, what are you both doing in a situation where you can talk to someone else and spend enough time talking that you end up falling for each other ? This simply does not happen when everything is as it should be.
Which leads on to Thing Two. If you were to walk away from your marriage now, and then things did not work out with Online Man, would you regret walking away ? If yes, step away from the keyboard, now. Then go and spend time with spouse, nurture the relationship you don't really want to lose.
If no, step away from the keyboard, now, then go and deal with current responsibilities and wind down the relationship you no longer want. Either way, your first responsibility is with the man that is already in your life.
BTW, I am not having a go at you. We were in the same position, 13 years ago. We asked the same questions of ourselves and each other, and made our separate decisions to walk away from the marriages we were in at the time.
And THEN we met up for the first time.
It's never ideal, falling in love with someone when you happen to be married to someone else. There are only two possible outcomes if such a situation that are reasonably fair and decent, and both involve someone being walked away from.
Only you know who that should be in your case.