Why is same-sex marriage being legalized in some US states?

The simple answer is that the politicians will tell you that it is "the will" of their constituents and they are passing the legislation in response to popular demand of the voters. HOWEVER, it is much more complicated than that and this is an an opinion and discussion question that cannot truly be given a definitive answer on this venue. Suggest you might want to pose this question on a chat-room, blog, or other "discussion-type" location. People who favor legislation that allows same sex marriage see it as a civil rights issue: equal rights under the law. Marriage in the United States and elsewhere bestows a multitude of civil rights upon the happy couple. Advocates believe that same sex couples in a committed relationship should have available to them the means to form a legal civil union that bestows those same rights.