Oh, you're Catholic! That makes it much easier. Sweetie, just talk to your priest. Tell him that you have been having tarot card readings and are worried about what they said. I am sure he can put your mind at ease that your future is not written, and that we all have free will to choose our own path in life.
I have seen some expose's on shows like Dateline and Sixty Minutes about people who "read" tarot cards, palms, crystal balls, horoscopes, etc. The way it works, and the way the good ones are able to make it seem like they can predict the future, is by getting to know as much as possible about the person, their job, their life, their family, the people around them, and then using their own life experience to try to predict what is going to happen. They just use basic principles of psychology and plain old research into the person's life. They also try to keep their predictions vague so that they are more likely to come true.
In your aunt's case, I'm sure she knows you very well, talks to family members about you, etc. So she is likely able to predict what might happen by simple observation. For example, it is likely that you or someone in your family talked to her about your job, your boss, or this other gentleman at work to her, and what she heard led her to believe this man has an interest in you. Lots of people are very good at reading human nature--some can be so good at it that it makes it seem like they are predicting the future, but, in reality, they are simply functioning as a psychologist.
My mom is one person who is really good at predicting, for example, whether a relationship will last, or whether a man is interested in a woman. She knew my sister's relationship with her ex-fiance would not last and "predicted" that he would call off the wedding. She turned out to be right. Is she psychic? Does she have some kind of magical power to predict the future? No, and I'm pretty sure she's never touched a tarot card in her life. She is simply good at observing people, understanding their personalities, and drawing on her experience with human nature.
I think your aunt may have a similar ability to observe people and understand human nature. I think she can give you good advice if that is the case, but I think you should try to talk to her straight up, leaving the cards in the box. Ask her why she thinks you shouldn't marry your FH and why she thinks this man from work would be a better fit for you. Ultimately, however, I think the others have given you good advice with regards to the fact that you should marry your FH if you are in love and that is what you want, and don't let the cards dissuade you. As I said, good luck, and if you are really in love with this man, don't worry about it! 
