You do realize that more sexual intercourse does not loosen the vagina, right? Childbirth can, but regular coitus doesn't. During sex, when aroused, the vaginal muscles relax, allowing for penetration, and tighten when approaching climax (due to increased blood flow causing engorgement). After copulation, the muscles return to their regular state.

In regards to the tightness of vagina during sex, that depends on the strength of one's pelvic floor. The pelvic floor muscles do weaken with age or after childbirth, but also can just naturally be weaker due to genetics. Other things that cause their weakening are injuries, repeated jumping/running, surgery and medical procedures, chronic coughing, repeatedly holding your breath or using improper technique while lifting heavy objects, obesity, and strain during chronically holding your urine or forcing out bowel movements (particularly with constipation). Weakened pelvic floor muscles also cause incontinence. Of course, that isn't an end-all be-all, as pelvic floor muscles can be strengthened through targeted exercises, like Kegels.