With hpv, does having unprotected sex regularly pass the virus back and forth between partners? like would a couple keep reinfecting each other?

No: If you are in a mutually monogamous relationship, you are not passing the virus back and forth (not "reinfecting" each other). Once someone is exposed to and infected by hpv, that person becomes a carrier.

No: Once you have had a strain of HPV you develop antibodies to that strain. In monogamous relationship you both remain carriers but are immune to re-infection by that strain. You could pass it to someone else or get a new strain from someone else.