A 14 day suspension is only issued for a first time offense if you used one of Riot's zero tolerance phrases, which includes (but not limited to) racial/homophobic slurs and death threats/encouraging suicide. This is because you should not need a warning that these are unacceptable. If you want a community opinion about your punishment, you can post your chat logs, and we can point out what exactly may have triggered this escalated punishment.
If your chat logs don't have anything that would be worthy of an escalated punishment, and this truly was your first punishment, then you may have a good case to argue that your punishment should be reduced.
Lastly, if this punishment does stay, then what Zombie Gerbil said about your next punishment is true, though blunt. Punishments are given out in an escalated tier fashion, and the next punishment tier after the 14 day suspension is a permanent ban.