Understanding foreign key not-null=true and inverse behavior in a zero-to-one relationship with NHibernate

I'm trying to get NHibernate to use the many side of a collection to manage a bidirectional association to model a zero-to-one relationship.

Parent Class and Map:

public class Parent
{
    private ICollection<Child> children;
    public Parent()
    {
        this.children = new HashedSet<Child>();
    }
    public virtual Guid Id { get; protected internal set; }
    public virtual Child Child
    {
        get { return children.FirstOrDefault(); }
        set
        {
            {
                this.children.Clear();
                if (value != null)
                {
                    this.children.Add(value);
                }
            }
        }
    }
}

public class ParentMap : ClassMap<Parent>
{
    public ParentMap()
    {
        this.Id(x => x.Id)
            .GeneratedBy.GuidComb();
        this.HasMany<Child>(Reveal.Member<Parent>("children"))
            .Access.Field()
            .Cascade.All()
            .Not.Inverse()
            .AsSet();
    }
}

Child Class and Map:

public class Child
{
    public virtual Guid Id { get; protected internal set; }
    public virtual Parent Parent { get; set; }
}

public class ChildMap : ClassMap<Child>
{
    public ChildMap()
    {
        this.Id(x => x.Id)
            .GeneratedBy.GuidComb();
        this.References(x => x.Parent)
            .Not.Nullable()
            .Cascade.All();
    }
}

The following code produces two inserts and an update:

var parent = new Parent();
var child = new Child();
parent.Child = child;
child.Parent = parent;
session.Save(parent);
session.Flush();

Notice the essentially duplicate SQL for the second insert and the following update:

exec sp_executesql N'INSERT INTO [Parent] (Id) VALUES (@p0)',N'@p0 uniqueidentifier',@p0='AA5A146E-E3F5-4373-B7A8-9EF301171401'
go
exec s