There is no rule restricting us in our love lives. We can date whomever we want.
Moi, personally is dating a "normal" person. (Although I have turned him darker. I just think I brought out his already hidden personality. He has confessed to me that he has always wanted to be, what is known as a 'Greaser'.)
Many Goths do. And many of us do date darker personalities. Examples: other Goths, Punks, Metalheads, Emos, etc.
If dating someone outside the scene remember, DON'T TRY TO CHANGE WHO THEY ARE!!!!!
First of all, are you not dating them because they are who they are? Trying to change them will only raise problems in your relationship. They will feel you do not love them for who they are and this will give a negative opinion on the Goth scene.
It is important in these relationships that you two understand and respect each other. You both can't be trying to change each other. An occasional dress up would do no harm if your partner is on board- don't force them!!
Other than that, you two should be fine. (Unless you two have issues that all couples do: jealousy, possesiveness, demanding, etc)