Time and time again I remind people that "fag" "that's so gay" and "queer" are extremely offensive terms.
Replacing the word "stupid" with a word or phrase that describes an individual is awful - it's mean and it's an inaccurate representation of the word, because "stupid" obviously does not mean "homosexual".
Most people who use these terms don't necessarily have the intent to harm someone else, but they are harmful. It's not funny and it's not cool. LGBT people are denied over 1,500 rights and benefits in the United States and many are executed in other countries every day. Gay jokes and anti-gay slurs are not funny.
Please, watch what you say and inform others who use these derogatory terms that what they're saying is harmful.
If you mean to say "stupid" - say "stupid". Don't replace someone's sexual orientation to mean that.