Without breaking the skin, but leaving behind bruises. can i cause my girlfriend any longterm damage? this is consensual biting btw

Physically no: The skin has great healing properties and i doubt you will leave permanent marks-- but frankly, if the bruising shows, people will think less of both of you. We all have certain freedoms, but the opinions of others do have consequences; her siblings or parents may see such marks as reason to dislike you, for example. That leads to family tension, that eventually will affect your relationship.

Biting skin: Unless your vision is as good as a microscope, there's always the risk that you will infect her with your mouth bacteria. Also if you are a carrier of a bacteria is resistant to antibiotics called mrsa, you risk a more serious problem. This kind of bacteria can cause your flesh to be digested and not respond to antibiotics. Obviously if you do this infrequently then the risk is very very low.