The Genealogical Forum of Oregon has placed a marriage index on its web site, listing marriages in Multnomah County from 1855 through 1904. Every marriage is included in the index twice, so you can search for either the groom or the bride.
The site contains only the index, not the original records. Copies of the original records are available by old-fashioned mail, or "snail mail." After finding the names in the online index, send a letter listing the names, volume, page number, and $5.00 to the Genealogical Forum of Oregon. A copy from the original records will be sent to you soon after.
Full details are available on the Genealogical Forum of Oregon web site at http://www.gfo.org/multcomar.htm.