In large part, I would say yes. Ellen Degeneres came out, vicariously, through her television character shortly before Will and Grace it the air. And it was a seismic cultural moment. Yet Will and Grace created very few stirs when it debuted. And yes, it paved the way to a growing number of gay characters on television(which went on steroids with Glee). But this piece also points out the other side of the equation that doesn't get mentioned very often; how Christianity is portrayed on television. The answer is horribly, when it is portrayed at all (Remember ABC's "GCB," which stood for "Good Christian Bitches?").
Imagine if gays received the stereotypical treatment on television that people of faith do. "L.A. Law" tried to address this in the early 90's with a young, Christian, female lawyer. And yes, there have been a few other series to present people of faith in a positive light. Very few. Most tend to be flaming hypocrites, cardboard cutouts of Christians as Hollywood writers imagine them to be.And to watch network television in the last decade would you guess that the gay population is somewhere between three and five percent?
Gay marriage is an idea whose time was going to come, sooner or later. Hollywood made sure it was sooner.