No it's not that. it's the onther way round, only in short amounts
a gf isn't gonna be there ALL the time, so it's settling for porn at that time.
If your gf's there and willing to have sex, yet you turn her down and spend your time looking at porn instead, then you've got a problem - porn addiction probably. Most guys wouldn't be like that tho -they'd just settle for porn when they needed to get off but couldn't have sex or whatever