My datasheet tells, that each port pin can individually be used as input or output. If you now take into consideration, that all port pins are always written or read at the same time, then this implies, that you must write a logical high state to the port pin, which is used as input, and this every time you write to the port, in order to write to the port pin, which is used as output. So, periodically writing '1' to the port pin, which is used as input, is not only allowed, but is a must, and this equally of the actual state of port pin, which is used as input, means even if this is grounded all the time.

Datasheet also tells, that during writing '1' to port pin, internal 'strong' pull-up is turned-on for a short period of time. But 'strong' is not so strong if you read the specifications table, where it's said, that this transient pull-up current is only 1mA at VDD = 2.5V. Of course, at VDD = 5V this will be some higher, but the time duration of transient current flow is limited, until the next negative going edge of SCL. So, this current will only flow for a short period of time, not being enough to cause any damage.

Kai