My penis is curvedown. is there any problem during sex? if yes then how can i straighten my penis?

There is a: Disorder which can cause curvature of the penis, but it is treatable. See your doctor and get a referral to a urologist.

No so no need: As long as your penis is erect, there's really no issue as to which way it curves as long as curvature is painless. Peyronie's disease can be thought of as painful curvature. Regardless, we have no good way to straighten out your penis. Again, as long as it's painfree, there's no problem during sex. Of course, you should always wear a condom to reduce risk of sexually transmitted disease/infection.