What should I do if my husband doesn't want my children to be around with my girlfriend?

He is demanding it be put in the parenting plan that our children will never be around my girlfriend because he feels it will damage them. However for over a year she had been around them with no issues until my daughter asked his girlfriend if she was gay. Now he feels my children are confused and could never understand a same sex relationship and wants me to put in writing that I will never have her around them until they are 18 years old.