You might also get away with a more gothic theme if you tell your family it's supposed to be 'Victorian' or 'Renaissance' or some other historical period -- that appeals to family desires for tradition and seriousness (whereas the idea of a gothic wedding may make some family members think you'll be sacrificing virgins or something!). A Victorian-Gothic theme is very appropriate if your family is rather old-fashioned in outlook. Most current wedding clothes are based on Victorian styles anyway, so things like tailcoats, corsets, and top hats won't seem so extreme.

Besides, at many traditional weddings I've been to, the parents and grandparents (i.e., the older generations) tend to sit out of much of the reception anyway. They would show up, greet people, dance with the bride and groom once, then politely retreat to the shadows or just go home. And this was a the type of weddings where "Super Freak" by Rick James was being played, not "She's In Parties" by Bauhaus!

This might take place at a favourite goth club or in a cemetery or in some other typically gothic setting. The ceremony might be a real ceremony (maybe performed by a friend with a Universal Life Church minster card) or a fake ceremony (done just for show) or perhaps a pagan ceremony (something that would be meaningful to the couple but not approved of by their families). The reception could be a completely uber-gothic affair with black decorations, elaborate goth-fetishy outfits, and dancing to goth tunes.