I'm going to throw out my girlfriend's ancient pants that she insists on still wearing
Why do you want to trash her clothes?
Because they are not suitable for the hot weather
There must be an attachment to these pants that gives her some kind of intrinsic value. Therefore, you shouldn't throw away something that belongs to your girlfriend. It's probably her favorite pants so if you throw them out, it will very likely result in a dispute and the relationship might fall apart.
Fuck her. She wears them all the time and I don't care.
You have no right to throw her favorite clothes away. If she finds out what you have done she may break up with you. Have you considered trying to find something similar that is new she could wear?