Even in the midst of excitement, it is not erect but it is curved. Dear Straight and Narrow, Like other physical characteristics, penises come in different shapes and sizes. Some are longer, some are shorter, some are broader, some are narrower, some curve to the left, some to the right, some not at all. You mention that even in the midst of excitement, your penis is not erect.
Erection is caused during arousal by more blood flowing into the penis than flows out.
However erect does not mean exactly straight as an arrow, hard as nails, perpendicular to your body, and parallel to the floor. Each person's erection is unique as well. It may only be your expectations that are a problem.