Liebestraum, in the version most people know it, is actually the third in a set of pieces written by composer Franz Liszt all under the title "Liebesträume" (Dreams of Love). They are based on three poems describing different aspects of love. The last one - the one I'm featuring here, also known as Nocturne No. 3 - is based on Ferdinand Freiligrath's poem about unconditional mature love. Love for a lifetime - that's certainly an appropriate theme for music played at a wedding!
Speaking as a pianist, I would say that if you choose to use this piece as part of your wedding music, plan on using the final A-flat major section of it. The entire piece is rather demanding to play (as Liszt's piano music generally is) with some virtuoso portions, and the gentle, romantic theme is probably the most recognizable and useful portion for a wedding ceremony anyway.