No, this is not cool. Louis C.K. and South Park were wrong, actually. There are a couple of things wrong with it.

If you're yelling fag at a straight person, nobody thinks you're yelling "Hey straight guy, you literally have sex with men!" because that wouldn't make any sense. What you're yelling is "You are something bad. You know, like gay people are bad." That's why you never hear anyone say, "Hey you cut me off, supermodel!" or "You spilled your drink on me, fireman!" because it's not a random word after that comma, it's an insult. So, yelling fag at people who aren't (necessarily) gay is of course not being homophobic...to the straight person you're yelling at. It's being homophobic to the gay people who may or may not have heard it.

Second, the whole premise that you can use slurs and it is up to the target of the slur to figure out if you mean it in the hateful way or the "fun" way is dumb. That's not how the world works. You can't go up to a guy in a bar and say, "Your girlfriend is ugly and your mom's a slut" and then get mad when they take it as an insult. It is not other people's job to look into your soul and realize that when you're saying something insulting that you don't mean it that way. I'm sure you think "it's just words", but you probably don't know what it's like to be on a public bus and hear murmuring of "faggot" from the seat behind you. Maybe you're a tough guy and wouldn't have worried about it, but I had a long thought about what my plan would be if this guy attacked me for being gay. He didn't, but that fear was still real.

If it makes you feel any better, this is not something that only straight people have to abide by. There's a great Dead Milkmen song called "Instant Club Hit" that I love. It's a funny song from the 80s that makes fun of goth culture, but there's a part where it says "art fag" over and over again. Now I know those guys are cool. When I saw them live in Portland they asked people to donate to a group called Safe Pride PDX. One of the members is gay. Anyway, I scolded my husband once for putting that song on the jukebox at a bar. Not because I think the song is really homophobic, but because some gay guy who doesn't know this band could be in the bar and all of sudden realize "Oh fuck, the song playing in this bar is saying fag over and over again!" and get nervous that they aren't safe. So we don't play that song on the jukebox anymore. And you can think of another thing to yell at people who cut you off in traffic.

https://www.youtube.com/watch?v=hVGhHdRZJZc