I'm going to ask to spend less time at home over winter break
Why do you want to spend less time at home?
I'm going to Portugal with my girlfriend.
That's great.  You can spend your free time however you want, it's your freedom as an adult. Your parents should be happy if you are doing things that make you happy, so it doesn't matter how long you spend with them over winter break.
They should be happy I have a girlfriend?
Yes, they should be happy for you if you are in a healthy and happy relationship and getting to go on fun adventures. Everyone has a choice in how they spend their free time.