Advice Column: My fiancee is a serial cheat

I am a 30 years guy and am in a relationship with this lady whom we stated dating from secondary school. We stated dating in 2004 when I was in SSS2 and by the time we got to higher institution,we had become an item although we went to a different higher institution. my parents like her so much but there is this thing with her and the opposite sex.

While she was in school, I caught her with her lecturer, during my youth service days I also caught her again. I served before her because my school was faster than hers then during her own NYSC , I also caught her once more then I swore that  we would just be bed mates and nothing more. my mistake all along was that I did not tell anybody. Now we are both gainfully employed and she is on my neck that we tie the knots. my parents have taken her side as they also keep pressuring me into taking her for a wife.

After much hide and seek, I told my parents that I am not going to take her as a wife as I already have this lady that I am dating on the side and I love her so much.

The problem now is that my parent want me to marry this serial cheat.

What should I do???