I have to get a rhogam shot every pregnancy, because my husband is RH positive, and I am RH negative.
If I do not get that shot, and I have an RH positive baby, some of the baby's blood is mingled with mine, and my body would start to produce antibodies to that protein since it is foreign.
If I were to get pregnant again, with an RH positive baby, my body would likely reject them, and they would die, and any further pregnancies would be in danger because of the antibodies my body would have against that RH protein.
Oh, and if you don't plan on getting married, then you don't have to worry about any shots related to RH negative blood.