I second YGirl's comment.
There is no "protocol" on whether or not girlfriends are allowed to sleep in wive's beds.
In our marriage, we have an agreement that we can have sex with other people in the house, but the bed is sacrosanct. In other marriages, they all share the bed together. In yet other marriages, some partners are allowed in the bed if the spouse has met them and is comfortable with that person in their bed.
I wouldn't want to date any man who would be offended at not being allowed in my husband's bed. That would tell me he does not respect my husband's boundaries.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker