In and of itself, I don't have a moral problem with simply the fact that the couple is married or that a client is having a relationship with a married woman or man. The issue of breaking up couples, particularly married couples, is complex and isn't really necessarily about "preserving the sanctity of marriage no matter what" or any naive crap like that. It's about the fact that relationships come in all shapes and sizes, and that sometimes breaking up a couple will cause harm that is not justified in the bigger picture. Let me give you some examples.
I have a client come to me to break up a married couple. The client is dating a married man and he kept on promising to leave his wife and never did. I do a divination that reveals that even if the couple were to break up, the guy still wasn't likely to marry the client he'd been dating, which was what the client wanted. And coaxing an early breakup in a marriage that was not quite ready to break up on its own causes problems that go far beyond the emotional state of the client who's left out in the cold. At the end of the day, I'm looking at work that will cause a lot of emotional turmoil for a lot of people, a lot of expense for a lot of people, and will still not result in a happy client. That's not justified work in my book. There are such things as less-than-perfectly-happy married couples who nevertheless desire to stay married for any number of reasons. That couple has to be taken into account as well as the client, especially if there are children or complicated financial/extended family concerns (one spouse's insurance or job is helping pay for the other spouse's parent's hospice care or something like that).
A client wants a couple broken up because he thinks the relationship is the obstacle keeping him and his love interest apart or from being in the kind of relationship he wants them to be in. But the love interest has no desire to be in the kind of relationship the client wants to be in. Breaking up the couple would not benefit the client.
A client wants a couple broken up and the couple is bound by religious marriage vows, which are essentially oaths. Breaking up the couple means fighting against the religious oaths the couple took to each other. This is a bad idea. If one member of the couple indicated they were ready to get out of the marriage and just needed some help, then that might be a mitigating factor. But if that hasn't happened, and the two people are not ready to completely break their oaths (even though they may very well be violating aspects of that oath), then the client is fighting an uphill battle.
There are many more examples. But hopefully that gives you an idea of why I have to do a consultations for stuff like this. A consultaton will get at what the issues are and give us space and time to talk about your case. This will enable me to make better recommendations (for instance, the best route might be to draw a new lover to the other spouse so they let go with less fight, or to provide an unhappy spouse with the financial or emotional or medical or whatever things they need to be strong enough to leave the marriage).