My ex girlfriend wants to be with me again even after getting pregnant with another guy's baby?

About a couple of months ago I broke up with my girlfriend, she got another boyfriend (who is known for sleeping with a lot of women), slept with him, got pregnant, and broke up with with him. Now she wants to be my girlfriend again because she says she loves me and misses the time we spent with together. She said she'll wants me to father her baby. I love her too, but its extremely painful to her carrying some other guys baby and I don't want to be its father, why doesn't she understand that? What should I do?
My ex girlfriend wants to be with me again even after getting pregnant with another guy's baby?
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