Honestly, homosexual people do not own exclusive rights to the word "gay". In case they're unaware, prior to just a few short decades ago, the word "gay" had nothing to do with homosexuality at all. It simply meant "happy". In fact, it was rather a useful word. You could have a gay party or a gay outfit. Now, a "gay party" would be one where everyone's homosexual, or at least pretending to be.
I really don't think there's anything wrong with the use of the word "gay" depicted in the upper portion of this image. I am not offended when someone says "that's gay". In fact, I use the phrase myself. And I think it's ridiculous for gay people to be offended by this. It isn't marginalizing any group of people. It isn't blatant homophobia. Banning gay marriage is blatant homophobia. Saying that gay people will destroy society--that's homophobia. But, taking a word and using it in a new way is not. It's simply one of the characteristics of language--it changes.
When someone says "that's lame", are they marginalizing people who don't have the use of their legs? No. They're using "lame" to mean something other than "paralyzed from the waist down". When someone says "that's stupid", are they marginalizing people of less-than-average intelligence? No. They're using the word to mean something other than "unintelligent".
Words have multiple meanings. That's all there is to it. And words change meaning all the time. The word "want" used to mean "need" and now it means "desire". The words "thee", "thou", and "ye" all used to be used for the second-person and have all been replaced by "you". (This means the definition of "you" changed.) Definitions change. Language is not static. It is fluid. Fifteen years ago, the word "google" didn't exist. Now it's a verb and a noun.
Someone being upset at the use of the word "gay" to be synonymous with "lame" or "stupid" (as described above, not in their original meanings) suggests to me that they do not understand the fluid nature of language, and the fact that some words (in fact, most of the ones we use on a daily basis) have multiple meanings. "Gay" can mean "homosexual" or it can mean "lame". There may even be some people who still try to use it to mean "happy". That's fine. There's nothing wrong with that.
But, I really do think that there's danger in hypersensitivity. Our culture is growing ever more hypersensitive. We are taught that to be "politically correct", there are certain things that we must word a certain way, certain words and phrases that we must avoid using, etc. And, I can see that there is some place for this. There are expletives and insults that I do not think should be used (at least, not lightly). For example, it would be inappropriate for me to use the word "nigger" in just about any context that I could use the word. Certainly, it would not be appropriate to use the word to refer to a person with black skin--and most definitely not to their face.
But, I think we often go way too far. And this is one such example. Don't be offended just because people decided to take a word that might, with one of its other definitions, describe you or a friend or someone you care about and use it to mean something unpleasant. Be able to laugh at yourself. Don't take life so seriously. Don't give yourself ulcers trying to make sure no one ever says anything offensive ever. It's a waste of time.
Yes, people use the word "gay" to mean unpleasant things. They might say that a high price at a concession stand is "gay", or that the dress code at their school that doesn't allow them to wear their favorite outfit is "gay". That's fine. It should not be interpreted as an insult to every gay person. It shouldn't be interpreted as a slur against homosexuality. It's simply another use of the word "gay".
This is also why I think it's ridiculous that one of the arguments of the Christian right, in defending its opposition of marriage equality, is that it would change the definition of marriage. Definitions change. Words change. The word "marriage" hasn't always existed. The concept of marriage hasn't always existed. It has changed. And it will continue to change. All of language will continue to change. It will never remain constant. And there really isn't any benefit in trying to pretend that it will or that it should.