I remember years ago, i dont know if its still in the songbook, there was a song about how "sisters were a large army for Jehovah", lol. Basically, it has been recognised for many years that the Borg is full of single sisters, and so much so, that they make up a "large army".
Its quite apparant, that the GB WANT an organization of a bunch of single sisters. They have been denouncing marriage for years, trying to keep the woman alone, and hence, reliant on the Borg. Its like the GB feels they can have much more, and easier, control over single woman. Also, as others above have said, the congregation LOVES to "use" people, and single sisters must seem like easier group to "use".
Once a woman is married in the Org, the "control" is shared by the Borg, and her husband. The Borg, then, loses complete "control", and god knows, they cant have that.
This is just another example showing how much of a high control cult the Witnesses are. The control is indoctrinated literally from infancy. You must sit still in a chair for 2 hours straight, when you are a TODDLER. You must not celebrate holidays, and have school friends, or join sports, or dancing, or gymnastics, when you are a child. When you are a teen, you cant think for yourself, or act on ANY natural urge, or dress how you want, or do your own hair how you want. Then when you are of married age, you are better off remaining SINGLE, and whether you are a "brother", or "sister", if you stay single, the BORG has their complete control of you, well into your adulthood.
But dont worry, Jehovah loves you and will bless you.