It’s you OP.

Several Reasons

One - you assumed your daughter would accept someone she clearly won’t. What were the circumstances of you and “Nelly” getting together? There is usually one I can think of that breads the much animosity.

Two- You chose “Nelly” over your daughters feelings. You and Nelly could have been together without being married and living together.

Three- Since you did get married, you have not put boundaries and restrictions on your daughter’s behaviour. Do you think this is acceptable if she goes off to higher learning and has to live in a dorm?

Four - Are you sure “Nelly” is as blameless as you believe?