I have problem with my penish.my penish is downword and curvature like banana.so any problem during sex with my g.f.

Peyronie's decease : This is a common disorder that increases with age. Here is a reference for you. http://www.mayoclinic.com/health/bent-penis/my00759/dsection=causes.

Curved penis: This condition is called peyronie's disease. It is a genetic disorder (ie., you were born with it) which causes excessive fibrous growth on one side of the penis, which cannot stretch with an erection, and thus produces a curvature that in severe cases can make sex difficult. You should see a urologist about this condition; surgery is possible.

Penile curvature: I am assuming that it has not always been that way and that may suggest peyronie's disease (see, e.g., http://www.Uroweb.Org/gls/pdf/14_penile_curvature_lr%20ii.Pdf). You should definitely contact a urologist for evaluation and guidance.