Adam Lasky and his fiancée wanted to keep costs down when they began planning their wedding last year. They decided to invite 75 guests—no cousins!—to a marriage ceremony in the chapel at Mr. Lasky's alma mater and to forgo a reception. They were planning to use the money they saved for a down payment on a future home.

The groom's family had other plans. For months, Mr. Lasky says, his parents and relatives begged him to have a traditional reception—and to invite more people. "Every week, someone said, 'You should really think...