Would having a curved penis make sex difficult for my girlfriend?

No, unless? No unless you have an extreme curvature. If this is the case, you may wish to discuss with a urologist. A curved penis may cause some discomfort if the female has a tilted uterus or other gyn problem and you happen to touch that area with your penis in the curved manner you have and depending on your length and girth of the penis. Both may need to be examined and discuss this with gyn &/or urol.