There is no such thing as ''the alternative genre''.. Alternative means different. Alternative genere(s) therefor would be anything different than what is currently considered to be ''mainstream''. This does not just apply to music but to everything. I remember when I was 15/16 me and my friends used to be picked upon by almost everyone in school for being ''alternative'', but some of the clothing/style we wore back then is now being sold in ''mainstream'' shops and is no longer considered to be ''alternative''.
It's just a very suggestive terminology that I would advise to just let go off all together. Music is beautiful in all its varieties and to simply define a bunch of it as ''alternative'' is an insult to the creative minds who made the music. And people who unilaterally declare ''They don't like alternative music'' just haven't reached that point in life yet where you start to realize wanting to classify everything in easily identifiable categories is not going to make your life any better.
Everyone is of course entitled to like what they want and dislike what they want, but I feel people should at least take the effort to experience things before declaring they do not like them =).