Instead of banning her, I think it's better to encourage her to develop a healthy relationship with the boy. My son is 10 but he has a gf already. We try to set boundaries for him. We told him what's wrong with his behavior, for example, it was wrong to ask for a girl's underpants, etc. Encouraging a normal relationship is better than creating a phobia of bf/gf relationship. You should ask your husband, "do you want your daughter to be unmarried for her whole life?"