Most Catholics in America do not feel obliged to follow the churches position on this issue. You can find many Catholics who are pro gay marriage or for whom this is just not a major issue that will effect their vote one way or the other. Only conservative Catholics would this have a great effect on, and they wouldn't be voting for Obama anyway.
Italian FM arrives in Iran to promote economic, political ties (Reuters)
from the article: Obama becomes first U.S. president to support gay marriage