מתני׳ האשה נקנית בשלוש דרכים וקונה את עצמה בשתי דרכים נקנית בכסף בשטר ובביאה בכסף בית שמאי אומרים בדינר ובשווה דינר
ובית הלל אומרים בפרוטה ובשווה פרוטה
The first Mishnah in Kidushin 2a, explains that you may marry a woman by spending money ("Kesef") and giving her a dinner. However, this is the opinion of Bais Shammai. Bais Hillel says one may only use currency such as a "perutah" (copper coin) or an item of equivalent value.
But even say that Bais Hillel would agree with Bais Shammai that a dinner is enough; as we see in Kidushin 46a:
מתני׳ האומר לאשה התקדשי לי בתמרה זו התקדשי לי בזו אם יש באחת מהן שוה פרוטה מקודשת ואם לאו אינה מקודשת בזו ובזו ובזו אם יש שוה פרוטה בכולן מקודשת ואם לאו אינה מקודשת
This Mishnah clearly explains that one may indeed sanctify his new wife by giving her a date. And the Tanna of our Mishnah says that it works as long as the date has the value of a perutah. And who is the opinion that uses the perutah? It must be that this Tanna is therefore in accordance with Bais Hillel.
So all opinions agree that one may marry a woman simply by taking her out to a dinner-date.
However, later poskim explain that the minhag Yisroel changed since then for two possible reasons,
- The Mishnah on 46a continues:
היתה אוכלת ראשונה ראשונה אינה מקודשת עד שיהא באחת מהן שוה פרוטה
"If the woman finished eating (on one of the dates) first, she is not betrothed..."
So it became the custom to have a ceremony first while the bride and groom are fasting. Now that they are publicly married, it doesn't matter who eats everything first when they break their fast in the yichud room.
- The first Mishnah in Kidushin on 2a continues:
וכמה היא פרוטה אחד משמונה באיסר האיטלקי
"... and how much? (does the groom need to give to marry her) just one perutah??! (The Mishnah is shocked, "lashon B'temiah")
and the Mishnah answers... "because of the fatty forbidden (foods) of the Italians".
So we see that since the original reason Bais Hillel promoted even a small coin as a betrothal, rather than going out to dinner, was to avoid the potential that the young couple would be invited to an Italian restaurant by some Romans; and who knows where that will lead?!!..
So today the custom is that we make sure the wedding is a publicly catered Glatt Kosher affair.