Well the other part of this concern is that you have to consider that a finger is smaller in diameter than a penis. A finger can easily fit into the mouth of your cervix, and honestly, that can cause bleeding quite easily. A penis head is much larger in diameter and will NOT fit into the cervical opening.

And yes, your cervix has an opening even if you aren't giving birth or having a period. Stimulation from a finger tip can cause enough dilitation that the lining that is built up in your uterus can shed just a little.