There are several reasons. First, for the past 20 years or more, the media has engaged in a full court press trying to "normalize" gays in America.
Then, of course, comes Hollyweird with their "gay agenda" programs such as Glee which seems to add a new gay character about every other week and The New Normal. And, finally, not to be outdone, the NEA has utilized the public schools to indoctrinate our children that gay relationaships are totally "normal" and don't harm anyone. However, what they have carefully ignored is that the children adopted or raised in gay relationships lean more toward gay relationships themselves than heterosexual relationships. It's a little factoid that the left prefer you not know.
Oh, and last, but not least, this whole issue is about ~3% - 4% of the population.
Squeaky wheel, meet the grease!!