Sticks and stones I guess.
When I was a kid I used “gay” that way before I knew it commonly refered to homosexuals. We also played games such as “Smear the Queer” again before realizing it refered to homosexuals. With other bigoted terms like “nigger” and “wetback” etc. we knew exactly what it meant and who it applied to at a very early age. I admit I used to use the word “gay” to mean stupid or silly probably up to high school. As I got older and realized the use of such words were hurtful I tried to stop using them. Being on the receiving end of name calling and harrassment in school, I realized that I might be having the same effect on others, unintentionally by using such words, that bullies intentionally put me through.
There may also be some regional differences too, as far as interpretation and the severity of the slur. For the most part, I agree that most of the time it’s intended to be derrogatory. But I don’t think that the intention is always to disparage homosexuals. (But then, I grew up in an area where “gay” with the traditional meaning of “happy” was still used.)
The OP asked if I ever use it in the context of “lame”, my answer is “no.” Whenever I hear it, it’s usually from another queer person who is referring to something that is gay.
The OP asked if I ever get offended by it and my answer is also “no.”