Tables are set with place cards, there are NO extra/empty seats at the wedding breakfast/tea/dinner, so crashers can't stay without being caught.
... there is no way that they could do some of the things they did, without being caught and summarily tossed out on their rears.
After thirteen years as a wedding professional... I hate to tell you that you are dead wrong on all accounts. The type of wedding you describe largely went out of vogue with the Edsel. Wedding crashing is more than just possible, and women FREQUENTLY bring the 'um-friend' to a wedding.
It is tragic. It is a direct sign that the person is trying to have more wedding than they can afford. If you invite a 17+ year old person you must expect they have a significant other. If you can't afford to invite both, don't invite them. It is tacky.
Not only have I given such a wedding, but have been to at least 20 such, in the past few years, in N.Y.C.,Conn., Cal., and Chicago.
And FWIW, I was talking about just WHY the premise of the TOPIC OF THIS THREAD, which isn't anyone here's wedding,nor ones that they have attended , is just ridiculous.
I went to a weddding this past weekend that had place cards for everyone. There were empty seats because some people didn't show up. I know when I have mine, there will be no 'crashers' because my mother will be in charge :-)