Hi, this is Petra. I do anal with my bf (nothing great for me) and sometimes hubby joins for a DP (which really does get me off). So for me it's the vaginal stimulation that makes a DP good, and yeah, the thought of two guys cumming in me. But you can see I'm already different from your gf and it is so much of a personal thing, I don't think you will get your answer by polling women here.
But sex is an adventure, so if you spend a little time and effort exploring and find you don't like it, so what? Maybe you'll find something else along the way that really is great for your gf and you. Good luck and keep us updated.