The organization spearheading the campaign in support of Maine’s gay marriage referendum says the pop-rock band fun. will donate proceeds from a Halloween concert to its cause. Mainers United for Marriage says the New York-based band is playing in Portland on Oct. 31 following a 16-country European tour. Mainers United campaign manager Matt McTighe says the concert six days before the Nov. 6 election will excite supporters and remind voters what’s at stake when they vote on a ballot question asking if they want to allow same-sex couples to marry. The band’s first hit, “We are Young,” reached No. 1 on the U.S Billboard Hot 100 chart. Its second hit, “Some Nights,” also cracked the Top 10 on the chart.