Hi,
@Mike1570, and welcome to Biblical Families. I agree with the advice above, and yet as a guy who was raised without hearing a lot of "the Bible says..," I like to be able to supply a line of reasoning from another angle.
I think a lot of people sense that something is messed up about monogamist culture and so they want to explore dropping its boundaries. But at the same time, because we have
equality drummed into us as the cardinal virtue, people end up saying, "Well, if a man can have this other thing, we can't really say a woman can't." In fact we're so committed to this
idea, women will defend it by pursuing relationships with a variety of men — friendships, close working relationships, all kinds of unsupervised interactions with all kinds of men — all, I would say, in the name of making the point that women are
just like men.
But they're not.
Nor are men like just women. In the name of these egalitarian ideals we go along with being placed alongside women in all our activities, and the culture causes us to never question these arrangements, and to even think we like it. And we assume that the resulting problems — from simple awkwardness, to deep and intractable social and economic conflict, to rape and worse — are just normal, the way things have to be.
None of us are really made happier by behaving in a way that isn't aligned with the way our bodies and minds are built to function. No matter whether you view that design as a matter of God's intent or something that's resulted from changes over time, life goes better if you work with the pattern rather than against it..
Getting involved with another man's woman doesn't work with how humans are designed. In the short run it may appear to work, but in the long run — nope.
The Bible says not to desire ("covet") anything that belongs to another man — including his women. Again, whether or not you take that as instruction coming literally from God, it captures something that people for thousands of years found to be sound guidance.
The woman of that other man may be saying it's all right for you to be with her, but it's a man's job to understand boundaries and live by them. If you hold the line and steer clear of her, then whether or not you already have a woman of your own, I wouldn't be surprised if the universe rewards you by opening the possibility of another just for you, and in a way that's better than you could have anticipated had things gone the other way.
I feel a little bad that all the advice here has to be so negative. There are plenty of fish in the sea, however, and I look forward to any of the various positive discussions that might come up in connection with one — or more! — of them in your life.