You should not be able to harm her cervix, although you can chafe or tear her vaginal opening if not enough lubricant is present. During vaginal sex the penis enters the vagina and stimulates the vaginal walls, gspot and may even touch the cervix. However, the cervix stays closed and only opens (dilates) during childbirth. However, the rest of the time it's plugged with mucus and is too small to allow a penis (or even a finger) to enter. During sexual arousal, a woman's cervix lifts upward and allows more room for the penis to enter the vagina but it does not open.

The vagina and urethra are separated and are separate openings. While the uterus and the cervix are accessible through the vagina, they are very well protected and even with a prolapsed uterus or cervix there is no way that you can penetrate them with just a penis or finger.