OK, so so far we know you really, really like how things are going on - that seemed to be your first line of defence, as though it would make you a better person than if you were only reluctantly sleeping with a 'taken' woman. You have a beautiful soul, drawn not just to her body but to the connection, she makes you feel wonderful, a profound connection, there's only emptiness without her, blah blah blah. None of this makes any difference as to the wrongness of your actions.
Now you're comparing this great love to day trading(!), talking about gains and losses (again, in the passive voice), and marital fulfillment. If someone does something wrong - does you a wrong - and, in justification, points to other people doing said wrong, would that make it ok? "Yeah, I mugged you, but do you know how many people get mugged in this city every day?". All this talk of gains and losses and risks - would you be so sanguine five years down the line, when you find out your SO has been seeing someone else?
If this woman is married, she will have made certain vows. If not, there will clearly be implicit ones. That's how it works. All the same, she's off jumping your bones whilst her SO is oblivious. If she no longer intends to keep to those vows, she should let her SO know.
Your rationalisations, as well as the very fact of your posting here, show you are troubled by this. Good. You should be. It's wrong. Stop rationalising and trying to justify, stop using the passive voice/abstractions to describe your actions and face up to what you're doing, and what that says about you. If you don't like what it says, change it.
Do the 'decent' thing: get her to leave her SO and have a clear, proper, out-in-the-open relationship with you. If you're right, this shouldn't be a problem. The love you have for each other is so pure and strong, her relationship with her SO is clearly falling apart and would have soon come to an end with or without your intervention, so make a proper go of it.
Or, better yet, walk away. This will come to no good. It's a rush at the moment, and it'll go on being so for a bit yet. Whilst you're caught up in the heat of the f*****g (and the plans/preperations/yearnings for such), you can tell each other and yourselves how special it is, oh if only she weren't 'taken', if only you could really and freely spend time together, and other such BS. Relationships like this are based on their very impossibility. It's where they draw their tension and intensity from. See, it's not just that you're sleeping with a woman who is 'taken', her SO perhaps oblivious, perhaps starting to pick up on certain signs - it's that the existence of the relationship entirely depends on her being taken, on her having a SO. Properly together, actually facing each other and not a romantic idealisation, the grind of the real, it'll fall apart.
Walk away, and maybe let her SO know. If he's a terrible man, she shouldn't be with him. If he's actually decent enough, he shouldn't be with her.
Anything else is cheap cowardice.
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Of course, it's probably quite a bit more complicated than that.
You know sometimes, between the dames and the horses, I don't even know why I put my hat on.