Reply #28: There's a lot of reasons...

[LostInAnomie]

... 1. Back in the 19th and early 20th century it was actually pretty rare for someone to have ever traveled further than 100 miles from their home. Many would live their entire lifetime within a 20 mile radius. This really narrows the number of members of the opposite sex you could meet. As our population became more mobile the option became more numerous and people that married their cousins were inbred hicks.

2. Inter-marriage was a good way to secure property along family lines. As laws changed about inheritance and social attitudes changed about women doing business there was no longer a need to make sure that property was passed down to a male member of the family.

I could think of a few more, but it's important to remember that most changes in moral outlooks are really based on people just deciding that the previous behavior was wrong. What actually changes the mores is changing economics and logistics.