What strikes me first is the question in your title. An interesting use of the passive voice. Not 'am I cheating', but 'is it cheating'.
The next is I wonder if you really meant to ask, 'is it wrong?'. Surely you'd know a single person can't cheat, and that whatever term may apply to your actions/role, it's unlikely to be a positive one. So then, your question guarantees the answer, 'no'. Whereas 'is it wrong' might bring, as it has when rephrased so, a 'yes'.
That you felt the need to ask any question on the matter suggests you are in some way uneasy about the morality of the matter.
We don't know the solidity or otherwise of the woman's relationship with her SO. Has she said anything to you about it? Has she suggested anything in the way of leaving him for you? It may be you've saved her from the shittiness of enduring a terrible relationship that is on its last legs, it being so being known to her SO, and that if you'd have met a few months down the line, she'd have been single and you would both have been free to pursue the relationship guilt free, the only difference with this way being you have something of a queasy beginning to look back on.
This doesn't seem like that, though. It will be over at some point, sooner rather than later. Either she'll move onto someone else, all the while staying 'taken', or she'll make a move to make it serious with you, and then it won't be fun anymore. And when she's with you, and it's no fun anymore, and she's got all these men turning their heads, it'll be your turn to hear her convincing lies as to where she was or why her phone was turned off.
As starvingartist said, it's unlikely you share a deep emotional connection. Rather, your circumstantial needs have happily (for yourselves) complemented each other, and create an intensity that otherwise wouldn't be there. So, you can steal time together, all the while knowing it's rare and fleeting, and you can have intense sex, and feel special, and perhaps muse on how it could be if her SO was out of the way (it would be terrible, as these things usually are - such relationships are built on their very impossibility, because of, not in spite of, the obstruction). You do not care for each other beyond what the other can do for you.
You speak of a 'deep emotional connection'. yet you haven't thought far ahead, are 'trying to make the most of it' and seem mostly impressed by the fact you were seduced by and are sleeping with someone of this (apparent) beauty.
You feel connected to the world because you've fallen for her. Rather, you've fallen for the fantasy of her. A beautiful woman, already 'taken' and regularly 'chased' seduces you - I mean, wow!, right? That makes you very special, no? A fantasy to life. Only it's not a fantasy, it's reality.
And as Stargazer43 said, how do you suppose her SO would feel, if he knew? This is his reality, too, one you're helping to make pretty rotten.
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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.