Anon
Wow, I think this is a question that should best be directed at somebody more qualified. I would recommend asking this exact question at

http://www.saint-mike.org/qa/default.asp

This is a brother who deals with many questions like this and an expert. It’s important to know exactly where your girlfriend is in regards to her Catholicism; is she a faithful Catholic, or just in name; where do YOU stand on your relationship with Our Lord, etc. I personally don’t think that sex is the biggest difficulty but differences of Faiths should be the biggest difficulty. I know from firsthand experience. The brother from this website counsels many who are contemplating inter-faith marriages. He is a former Protestant Minister and I have seen him deal with these types of issues and if you are really serious and wanting to do the right thing, then best get advice from experts rather than different opinions which could probably confuse you more than help.

I’ll be praying for you.