Can I suggest that we go to a vegan restaurant?
Why would you suggest going to a vegan restaurant?
Because my girl is a vegan
It's nice that you want to stick up for your gf and do things that she likes. This will make her very happy.
Nah, I'm just doing it because I know it'll make her happy, I actually like to eat meat.
It's completely fine if it is just for her, because like I just said, doing nice things for a person you care about to make them happy is a very important part of life. You're doing a good thing right now with this.