"A" is what's called an "Article", normally followed by a countable noun.
If it looks wrong when you read it, it probably is. As in Jim's "a mature what?"
GG's
"I Own a Mature Porn Site"
I (pronoun) Own (verb) a (article) Mature (adjective) Porn (adjective)Site (noun).
btw- I made an soccer goal yesterday!
See, it just doesn't make sense. Then again, most of the time, neither do I.