Are you the other woman? Even if you don't think so, if you are dating a married man you are. Not that long ago this situation was totally taboo but now it is more common. There are even websites where you can go and find a married person who wants to have a relationship outside of their marriage. The times they are a changin'.
If the acceptance level for dating a married man has risen what it entails is still the same. You are the woman waiting for it to be her turn. Will it ever be? Who knows. We have all heard the "He is leaving his wife" line and we all know that most of the time this is not true. If it is true then you have become the woman who has ripped apart a family. This type of relationship brings with it plenty of complications so be prepared.
Morals aside the reasons pushing you towards pursuing this type of relationship can be dangerous. If it exciting to you because it is forbidden then this is a problem. This could lead to you dating a person you would normally find repulsive as your judgment is impaired by the rush you get. If you are doing it because of low self-esteem then you are not making the decision with your eyes open.
When it comes to matters of the heart only you can judge what is best for you. You have to decide what is best for you at whatever stage you are in your life. Separate yourself for the moral issue at hand and make the decision based on complete honesty. If you still decide that you want this man then you've made the decision. Love sometimes comes unexpectedly and you can't always control it. If after an honest look you see that the relationship is not healthy for you then walk away.