Then it would be fitting for people to also stop using the term "that's lame" because it could be seen as offensive to those who are physically challenged or disabled? 
And by your reckoning, they would then also be using the term "that's lame" under the full understanding that it's roots lie in causing offence to disabled people? 
Is it not possible that people use a derogatory term simply because it's commonly used as derogatory? To which i imagine your response would be “and that is exactly the problem”. But the solution would be to start punishing people for what they think, except, people saying ‘gay’ as a derogatory term AREN’T thinking “aha, i think that person is bad, and being gay is bad, so i shall call him gay”. 
My point is that your highlighting something that isn't actually a problem. As i said before, my friends say things like "that's gay" yet have they been in any way unaccepting to my homosexualty? No. On the contrary, since i’ve come out several of them have noted how they’ve become far more open and accepting to gay people in general. The fact that they are completely comfortable with my sexualty, and they still use terms like that, only goes to show they don't do so with any concious underlying homophobia. The same thing holds true for most other people, in so far as if somebody’s going to insult you for being gay, the terms ‘faggot’, ‘shit-stabber’ and ‘batty-boy’ are far more likely to be used that ‘hey, you’re gay’. THOSE terms are offensive, and almost solely used as homophobic, why not complain about them? 
Girls as young as 9 can be executed in Iran for being lesbians; in the last 30 years over 4000 people been executed there for being gay. In Iraq “Homosexuality is seen as so immoral it qualifies as an ‘honour killing’ to murder someone who is gay – and the perpetrator can escape punishment.” But damn, in the UK ‘gay’ is a derogatory term.

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