Learn English – Is it offensive to call a redhead a “ginger”

This is a touchy and complicated issue. There is no simple answer. I'm gay and I probably would be far more offended by the use of "retarded" than by the use of "gay" that you're describing. The problem is not if you are offending anyone but if you might offend someone, and where and when. I personally don't mind people using the word "gay" that way, in great part because I think it's stupid, but not directly harmful; I think that is more damaging to the person saying it than to the concept of being gay. (Equating "gay" to something bad won't make being gay bad. South Park uses the word "gay" that way. When Cartman calls lame things "gay", Stone and Parker are mocking Cartman, not gay people.) I'm offended by the use of the word "retarded" for the same reason: as I am not retarded, if I were not offended, I'd be latently approving of the word being used that way.

But, in short, by using these words, you're always in danger of offending people, even if you haven't so far. Imagine: you meet someone you're getting along with. Let's call them person A. That person is with a friend of their own, let's call them person B. Your friend is with you. Your friend uses the words "gay" and "retarded" the way you described. Person B is offended by these words, either because a relative of his is retarded, or gay. Person A can offer you a job, or a piece of advice, or help you move, or anything of the sort. Person B later tells person A, "I don't like the sort of people that guy," (meaning you), "is hanging out with." Person A is now far less likely to help you.

Is it really a risk you want to take? From your question, it appears you are rather young. That leads to another issue: do you want to get used to that language? If using those words that way is not an issue now, do you want not to notice it when it becomes an issue to do so?

Think of bicycles and helmets. Someone might say they don't see the point of wearing a bicycle helmet because they've never come close to having an accident, but they wear them anyway. Maybe the two are related. Maybe the type of person who is so careful they've never come close to have an accident is the same type of person who wears a helmet. When you're in the process of falling down, it's too late to start wearing one.

You've never had a problem with that type of language and you probably won't until it's too late to do anything about it.

I don't have much advice to give, just a little. Don't try to change others. It generally fails to change anyone and alienates those who try to make others change. But don't change yourself either. You seem to think there's something wrong with using these words this way, despite not seeing much evidence that there is. My advice is: don't wait for the evidence to come along. Stay wary of that kind of language and don't start using it. If one day you run into a situation where it's clear that you're right, it will be too late to do anything about it. And if you never run into such a situation, what do you have to lose by being ready for it? The fun of using a few offensive words? I think it's a pretty good deal.

Before getting to the main answer, a few points that may be interesting to you (though I realise they're not actually part of your question!):

1. In the UK - at least in some circles, the word cunt is an insulting word for a man, never a woman: neither I nor those that I've asked have ever heard it used that way, though as the comments below attest, this usage does exist.
2. Of the others, the odd-one-out to me would seem to be bitch: I'm not sure about US usage of that word, but to me the others all have a connotation (to a greater or lesser extent) of sexual impropriety. In the UK at least, bitch doesn't have that association - it means something along the lines of woman with an unkind, evil or spiteful personality.
3. The words skank and ho are recognised - especially from rap music - but are definitely felt to be American imports. All of the others are fully accepted as native :)

On to the main question: it's probably worth distinguishing between the degrees of offensiveness of (abusing terminology slightly) use and mention of the words: that is, some words can be happily used in polite society, but one would be highly offended to be called one - whereas with some words merely saying the word (regardless of who it is applied to) is offensive in of itself.

Having drawn that distinction: mention of the word slag - while it's clearly to be avoided when on one's best behaviour - is unlikely to cause much offence; that would put it on a roughly similar level to slut or perhaps skank.

However, using the word slag to describe someone would be a good way to start a fight. It is probably at a similar level to slut: perhaps slightly more offensive, if only because slutty/sluttish can refer to general demeanour (in dress, speech) - and indeed can occasionally merely mean "untidy, slovenly" with no sexual connotation - whereas slag is unambiguously referring to sexual behaviour.

It's worth mentioning the word is used a lot in comedy - perhaps the earliest was in the comic Viz (decidedly not a comic for children, and never noted for its reverence); here's an example in Harry Enfield's sketch show; and more recently in Gavin and Stacey. (Using the word for men isn't common except in reference to this show!). You can probably see - especially from this last example - that this word can be thrown around among close friends in a humorous way - but I would strongly advise against doing so unless you're very sure of what you're doing, since the line between banter and offence is a very narrow one at times :)