American Muslims don't share the attitudes of the Islamic State, and so treating them with skepticism, generalizing about them, and also expecting some sort of collective responsibility that all Muslims must be held accountable for the policing those who share the faith, doesn't seem any more helpful than more gun control regulations.
American Muslims are more likely to support gay marriage as they have been exposed to a variety of cultures and opinions. While they may not agree with it in their own religion, they are more likely to see different views and opinions alongside their own and form their own conclusions. Evangelical Christians are too set in one frame of mind, that homosexuality is a sin and that marriage is between a man and a woman, to support gay marriage.
There are extreme factions of any group, especially when it comes to religion. Unfortunately, the most extreme groups often get the most attention, despite their not being truly representative of the larger community they are correlated with. Islamic terrorist groups are often painted into the same group as peace-loving Muslims in the minds of Americans. Fear and ignorance often cause Americans to have a knee-jerk negative reaction to Muslims; however, most true Muslims, especially in America, just want to worship in peace and truly love their neighbors, regardless of religion or sexual orientation.
I am not sure that there is any accuracy in saying that American Muslims are any more tolerant than Evangelical Christians. Most religions are opposed to homosexuality, with some being more strict than others. I think there are people in every religion that are extremely against gays, just as there are some who still love and embrace them, even if they don't follow the rules of their religion.