What would you do if you find out that she is dating someone else?

Let's say she lied about one person she has been dating recently, without going to bed with him. She tells you she feels something for this guy but she is in love with you, she choose you and wants to keep this solid long term relationship you and her built up.

What would you think? Would you be hurt by this lie and leave, or not?
What would you do if you find out that she is dating someone else?
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