casarse
Casarse con — you marry 'with' someone in Spanish. The preposition and the reflexive are both non-negotiable.
casarse
To get married. Always reflexive and always followed by con.
Se casó con su mejor amigo.
She married her best friend.
Casarse con + person. Never 'casó a'.
Nos casamos en mayo.
We're getting married in May.
Plural for the couple. Reflexive pronoun nos.
¿Cuándo os casáis?
When are you getting married?
Vosotros form, used in Spain.
estar casado / casar
Estar casado is the state; casar is to marry someone off (an officiant).
Estoy casado desde 2015.
I've been married since 2015.
The state of being married. Estar casado/a. Not 'ser casado'.
El juez los casó.
The judge married them.
The officiant does the marrying. No reflexive.
Están recién casados.
They're newlyweds.
Recién casados. Also 'los novios' = bride and groom.
Tricky bit
'Me casé a ella' is a classic mistake. Use 'me casé con ella'. Also 'me divorcié de' (not 'con').