It's a good question isn't it? Well, how about phrasing the question the other way around. Who would be in favor of discriminating against Gay, Lesbian, Bi-Sexual and Transgendered folk in employment? And why?
Actually the people against ENDA tend to use the argument that sexual orientation is a choice and therefore shouldn't be a protected class. Of course, religion is a choice too and it's a protected class so I don't much see the difference.
There is an argument that protecting GLBT people would put pressure on freedom of association employment for organizations like the Boy Scouts or a religious institution. I would be in favor of a religious exemption being added to the law because I think religions should be able to teach and believe as they choose. (Of course, everyone knows I'd rather see all religion abolished and that would solve the issue but I'm willing to be fair and compromise.) As for a freedom of association employment for non-religious organizations like the Boy Scouts...I think those type of arguments are just nice words for institutionalized bigotry.
The truth of the matter is ENDA is designed to protect the employment of people who are fired because their bosses are bigots, just like the protections for people of a certain race or gender or national origin or religion, ENDA is designed to protect another minority. The majority, by definition, needs no protection.
As we look back on the Civil Rights Act of 1964 we might find it hard to understand why such an Act was needed...why did Americans fire or refuse to hire someone because of their race or gender or religion? IN 2007 such a things seems sooo foreign to us. But we all know the answer...prejudice (i.e. bigotry). We think we've eradicated this disease but we haven't. Despite the CRA of '64 there are still people who don't like women or don't like black folk or Spanish folk or Jews. I grant you we don't see it as much today as we did in 1964 and we think it's because we evolved. Perhaps we have, but, in point of fact, some brave politicians in 1964 passed a law the FORCED the country to treat it's citizens fairly in employment and housing and voting. In 1964 many people weren't very happy about this law but we got used to it and we evolved becasue the government forced us to evolve. Sometimes the government has to pass unpopular laws for no other reason than it's the right thing to do.
I believe ENDA does the same thing for GLBT folk today that the CRA did for Jews and blacks and Hispanics and women in 1964. As the country and its people evolved, forceibly or voluntarily, this is the step that couldn't be taken in 1964...we weren't ready then...we're past ready now.
What will it say about our country if were still not ready to protect fellow citizens regardless of how we may personally feel?