A 17th century groom shows dandy clothes.   
A 17th century groom shows dandy clothes.

The reserved, stately dress of the groom in Catholic weddings was not always the custom.

Not much is known about what grooms wore in the ancient Christian world. Sculptures show couples donning what appeared to be their best togas.

Up to and during the Middle Ages, weddings were considered family and community events, not church or public affairs. Though a clergyman was on the scene to bless the couple, it was only later that marriage became a sacrament of the church at the Council of Trent in the 17th century. Grooms from lower and middle classes would wear their Sunday best and wealthier men would put on regalia fitting their station in life.  

During the Renaissance and into the early 19th century, grooms of means would embellish themselves lavishly with furs silk and velvets. They wore jewelry of silver, gold and gems.

The Victorian era ushered in a new reserved manner, even in Catholic countries like Italy, where sober black suits became the norm in the western world. Some trace the trend to the wedding of Queen Victoria herself, during which Prince Albert eschewed the usual royal finery and instead wore a field marshal's uniform. As the humble prince hoped, the queen stood out as the most important person of the wedding day.  

Thus, the groom's dress became an act of respect for the bride.

These days, the time and type of the wedding dictates the etiquette of groom's attire. Here is advice from brideandgroom.com:

Formal evening wedding — black tuxedos; black tails.

Formal daytime — gray pinstriped pants, cutaway coats or short jackets in black or gray.

Semi-formal evening — dinner jackets; dark or light formal suits (depends on season).

Semi-formal daytime — gray or black strollers; formal dark suits.

Informal wedding — dark business suits; white or natural jackets or suits in warm weather.