You're not married, so at this point your money is your money and her money is her money. If marriage is not on the horizon, it seems that you might just mind your own business about what either of you contribute or don't, just as I hope you would for everyone else in the parish.
So let's make this about whether your views on money are compatible enough to consider marriage. That is a conversation worth having.
In contrast to parishes that are always asking for contributions to the many self-improvement projects at the parish itself, I have actually heard a pastor say in talk on stewardship that a Catholic's tithe to charity should not all go to his or her parish directly, but that a certain amount--he suggested it be no less than half--should go directly to Catholic organizations who are known to obediently turn the tithe into works of mercy or that use the money judiciously for the greater spiritual advancement of the Church. IOW, your girlfriend may be right.
I would suggest this as you start this conversation: Realize that how much you give and where you give it are two very different things. You cannot simply give this money blindly to anyone who puts a hand out. You have a responsibility to give away your money according to the dictates of charity and justice. This will almost always include a portion to your home parish, but that is *almost *always. The fraction due to your home parish will vary greatly, as well.
Your girlfriend may only be concerned that giving money to this parish in the amounts that the parish is soliciting violates that standard. That is, she may think that the parish spends excessively, far beyond what the proper solemnity of worship demands or that far too much goes to things outside of the obligation to provide for the needy, educate, evangelize, and so on. If the parish is getting too much money already and using what it gets badly, it is OK to give to the Church in other ways.
Now, I am not against splendor in the appointments of a church. When it gives glory to God, these enhance the worship of the rich and the poor alike. The Basilica of the National Shrine of the Immaculate Conception in Washington, for instance, is a church with splendor that almost has to be seen to be believed. Nevertheless, it is a treasure that enhances the worship and witnesses the faith for anyone who goes there, rich or poor. Having said that, if a parish has let its spending get out of balance, so that comfort of parishioners or the splendor of their surroundings is seen to at the expense of comforting the poor and needy or the other work the parish is obliged to do, your girlfriend might be right. The parish may be violating the demands of charity and justice in how it spends and how much it asks for.
The other question, though, is whether she thinks that her material wealth is at the disposal of God's purposes. This is the question that will illuminate that: If your parish wants too much, then how shall the money that is beyond your needs be spent? If she wants to give it to Catholic Charities or St. Vincent dePaul or the diocesan seminary or whatever, then obviously, this is not a matter of her selfishness competing with the needs of the parish. She is only being prudent, an admirable trait in a Catholic.
The faithful are obliged to obliged to assist with the material needs of the Church, each according to his own ability: this is a precept of the Church. This does not imply that you have to give the money to a parish that is not making use of the money it is already getting, which it has in excess of its legitimate needs. If that is the situation in your parish--that is, if you have determined this as a fact, not just presumed you know it by outward appearances-- then you may legitimately contribute the money you are obliged to contribute to other needs of the Church.