Hey remember, that love is blind. I don't think that it matters that they have an alternative life style. Marriage is marriage. They love each other and when thy get old they will cherish the images you create. You should work from that angle. Capture the story of the two of them. Where the met, first kiss, first dates etc. Capture the details of their romance if possible. Every girl, punk or not wants some sort of remembrance. They are just built that way. If you understand their culture and what they are about, then document that and incorporate that into the wedding imagery.
Good luck,
Les
