What is the meaning or symbolism of people giving Money to Tweek and Craig for being gay?

Are the creators trying to say that in liberal society, you can be rewarded for being gay? A complete reversal of what was the social stigma of being gay, that now you can rewarded, which crosses the line of equal rights?

That's not really the message. You're close, but there's a nuanced difference here. Your interpretation relies on society actively wanting to reward people for being gay, but I think this is much more subconscious, born from a desire of making sure you don't come across as a homophobe, and therefore taking the opposite behavior (accepting homosexuality) to the extreme.

The real message is closer to saying that people are so very intent on showing how accepting they are of gay people, that they pretty much "make" Tweek and Craig a gay couple just so they can be supportive of them being gay.

Everyone mindlessly assumes that Tweek and Craig are gay, because they are no longer inclined to want to make sure that it is the case (that could maybe come across as being apprehensive of homosexuality). Because they avoid fact-checking (due to not wanting to be called a homophobe), they simply accept the rumors, and are only worried about formulating the correct response to the boys being gay, which they have mindlessly accepted to be true.

South Park of course takes this idea and runs with it, making it more overt and blatant as the plot progresses. This brings us to the quote:

"We're just going to go ahead and give them some money and let them go."

Mackey wasn't arguing that gay people need to be (financially) rewarded. It just the same argument as before: the school was so intent on showcasing their acceptance of homosexuality that they even raised funds, before ascertaining whether the claims were correct to begin with.
This suggests that the school either raised the funds without talking to Tweek and Craig (who both explicitly denied being a gay couple), or that the school dismissed what Tweek and Craig were saying (and mindlessly assumed that the boys were afraid to tell the real truth).

Either way, the point still stands that the school had an idea and ran with it without factchecking. And this is true of all the adults.

This plot device has been an often recurring plot device in South Park. Someone gets the wrong idea, and everyone starts behaving this way, partaking in the same crazy behavior. Even when directly contradicted, the people are incapable of listening to reason, and instead prefer to continue their crazy behavior.

Another great example of this is Two Days Before The Day After Tomorrow.

• The beaver dam broke.
• Global warming must have done it.
• We let global warming happen.
• We didn't listen!

Even when Stan explicitly confessed to having broken the dam, everyone was so caught up in their self-servicing "we let global warming happen" attitude, that they kept interpreting Stan's confessions as a symbolic gesture, really meaning "we all broke the dam".

And that's the point of this plot device. People are so caught up in their own world, that they are oblivious to the evidence that's right in front of them.

The same is true for Craig and Tweek's relationship. Tweek and Craig themselves were denying the existence of the relationship, which should be irrefutable proof that it's a baseless rumor.

But instead, everyone is so caught up in their self-servicing "we accept homosexuality" mindset, that they ignore the facts and simply run with the idea that they have.

I think the adults are trying to buy their way out of an awkward conversation on a modern social topic without having to admit their ignorance. "Atta boy!" or give'em cash are the only positive tools of support they have.

I think it's funny that the adults all default to the same method of avoidance.

People tend to forget that South Park is a satirical show which draws heavily from reality and the makers really like to point out certain trends and fads. Since Season 19 also breaks with the formula of the single-story episode, it doesn't make much sense to watch a single episode without acknowledging what happened in the earlier episodes.

The whole point is that the people of South Park don't like their town being depicted as a predominately white town full of backward racists with shotguns that slap their wives (or "the shitheads of America, Inc."), as it was depicted by Jimmy Fallon in S19E03.

In order to "become more socially conscious", Randy Marsh proposes the idea to get a Whole Foods Market into town, as it would "instantly validate us as a town that cares about stuff".

This leads to the creation of SoDoSoPa ("We're gentrifying. It's all good!"), which spawns CtPaTown, finally creating the appropriate environment for Whole Foods.

All those actions cannot really be considered as "socially conscious", because they just superficially "throw money at the problem" instead of making the real changes that are necessary to reach their goal.

In the end, it's not important if Tweek and Craig are really gay (as there's still plenty time ahead for development for the average ten-year-old). The Yaoi pictures where just another chance for the grown-ups to come across as progressive. Since they still don't know how to deal with "modern problems", they just keep on throwing money at them.