No, you would not be considered his guardian unless you applied for that through the courts and a court granted you the that position in an order.  Now, if he is under age he may need parental consent to marry you.  The laws vary from state to state but that is pretty standard.  Once you have parental consent and you marry you will be husband and wife.  Now, just because he is legally married does not mean that he is legal.  He still can not do what an 18 year old can do, like vote.  And neither of you can do what 21 year olds can do like drink alcohol.  Good luck to you both.