By sexually aroused, I assume you mean that your phallus swells. If that is the case, then it is normal. In the act of meditation, you are increasing the circulation in your body, including the phallus. Also, the rising of the phallus is actually what happens when the muscles in that area relax. So you are doing fine. No reasons to worry.
Now, if you are talking about sexual images racing through your head while meditating, then it sounds as though you need to focus more and that your mind is working through an issue.