I'm sure all of us have done this at some point in our lives, i just do it more often than others. I take extra napkins, plastic bags from cafes and save them for later use.

I personally, have never done this. In fact, if I explicity tell the waiter/server to not give me paper napkins if I already have my own, or plastic bags, if I already have my own bag. I also return unused paper napkins when I'm done with me meal, for example at Mcdonalds.

Basically, it seems like you're assuming that since you think this way, everyone else also thinks this way, only difference is to what extend. I disagree with that line of thought. It is entirely possible for someone to not do something you deem as common sense or human nature, or to reject your mindset completely. I.E, You think it's fine to take extra paper napkins, and in fact, everyone does it. But someone else might completely disagree and in fact feels that extra paper napkins should be returned if not used. Or there are might be people who actively discourage it.

And that is where I think, lies the root of your problem. You might be assuming that your behaviour is an indisputable justifiable action. When in fact, it's just a matter of preference. I'm not saying your wrong, I'm just saying that it's not something that everyone does universally.

You and your gf basically have different point of views when it comes to being frugal. You might want to consider thinking about how this might affect your relationship, whether it's a dealbreaker for either of you, and whether you should continue with the relationship.