Google is your friend. Only 13 States plus DC outlaw discrimination in employment based on sexual orientation. An additional 8 states outlaw discrimination based on sexual orientation in public employment (i.e. government employment) only. There are no federal laws banning discrimination based on sexual orientation. The Supreme Court has ruled that sexual orientation is not a protected class.
In most of the US, it is perfectly legal to fire someone for being gay (or straight, or bisexual or whatever).
See:
http://www.lambdalegal.org/cgi-bin/iowa/states/antidiscrimi-map
Lambda Legal. I can't imagine why I've never consulted an unbiased source like that before. Thanks for the heads up. Oh by the way Modernman....YOU'RE FIRED!!</sarcasm>