If a woman is so tight that sex is impossible, it could be a case of vaginismus caused by anxiety or a history of sexual trauma. I had this problem when I was with my ex because he was emotionally abusive and made me uncomfortable. I’m not saying this is the case with your experience as many factors may cause it. I’m just saying much of the time, “tightness” has a lot to do with nerves and/or dryness. When a woman is relaxed, her muscles will also relax, making for easier penetration.
Also, typical penis-vagina sex does not make a woman “loose”. That’s a myth believed by high school boys and girls, meant to perpetuate shame about female sexuality and encourage women to stay “pure”. Sorry guys, but unless you’re part thoroughbred stallion, your junk just isn’t big enough to do that kind of damage to us down there. Vaginas are amazingly tough, designed to take a pounding. In fact, using them often may even improve our “tone”. ;)
As for childbirth, that’s more true for some than others. Depends on the number of children, she’s had, how big they were, her age, and whether she had an episiotomy or not. Practicing kegels can also help maintain the tone of the PC muscles and improve sexual response. Of course, having children would have no effect on the vaginas of women who have had C-sections. (Which, by the way are awful)
So I would say there’s no reason for any nulliparous woman to be “loose” – whether she’s had 0 sexual partners or 50.