Dear Scout,
A cleric (bishop, priest or deacon) who attempts marriage (even just a civil marriage), incurs automatic suspension (Canon 1394). This means that he is prohibited from all acts of the power of order (celebrating Mass, administering the sacraments), all acts of the power of governance (the care of souls) and the exercise of all rights or functions with an office (administrative duties).
Such an act is a serious sin and can give grave scandal. Whoever cooperates in such an act sins seriously as well.
All sin can be forgiven if the sinner is sincerely repentant.
Father Vincent Serpa, O.P.