I understand where your GF is coming from - I believed the same things about 3 years ago (and for a long time prior to that). Here are some points:
If she believes the Bible is the inerrant word of God, it’s pretty clear about homosexuality being sinful. People have the freedom to be happy, but they do not have the license to sin. We should not confirm people in their sinfulness (which also applies to those having pre-marital sex of any kind).
If she does not believe that the Bible is the inerrant word of God, you may want to start here rather than starting by arguing against homosexuality or abortion. If she will accept the Bible as the inerrant word of God, the argument should be much easier.
As Kay Cee said above, Peter Kreeft is an excellent resource. Please visit his site, www.peterkreeft.com, and listen to the audio / read the essays. It’s pretty logical stuff, and he seldom relies on the Bible for his arguments. You might find him more useful if you are unsuccessful with the above 1 or 2.
Here’s an example of his argument against abortion:
**Moral Premise: **It is always morally wrong to kill an innocent human being
**Factual Premise: **Abortion is the killing of an innocent human being
**Conclusion: **Abortion is always morally wrong
Definition of Terms:
**Kill** – to, by voluntary and willful conduct, take positive action (as opposed to inaction or omission) by which death is forced upon another human being
**Innocent** – being undeserving of death; having no personal culpability; having done nothing by which to justify killing
**Human being** – a biological entity possessing life in the species of Homo sapiens
Life – the property or quality that distinguishes living organisms from dead organisms and inanimate matter, manifested in functions such as metabolism, growth, reproduction, and response to stimuli or adaptation to the environment originating from within the organism.
Underlying Legal Premise:
**Legal Premise: **The purpose of Law is to protect people, specifically to protect innocent and weak people from guilty and strong people; consequently, an immoral killing, which is the killing of the innocent and weak by the guilty and strong, should never justified by the law.
I haven’t found a better way to argue it or another way to see it. Furthermore, Peter Kreeft examines all of the possible objections to his argument, and knocks them all down. Truly, his are excellent lectures.
Peter Kreeft also takes an ecumenical approach to religious arguments, taking a stance that could be held by a Protestant as well as a Catholic. This should make things much easier to argue.
Hope this helps.
If you want more arguments against homosexuality or abortion, either start a new thread or PM me. As for the GF…well…the late Holy Father called us to a New Evangelization. I’m not convinced that inter-religious relationships aren’t a significant part of that.