The three main causes I believe women use profanity more now:

1) Confirmation Bias - When looking back on past events, humans tend to forget things that don't agree with them, and remember things that do agree. When you think back to music of the 80's, you only remember the one hit wonders, often that are still played today, and forget all the trash that was played on the radio between hits. There is still trash today, and when we look back on this decade we'll mostly remember the good ones and forget Kevin Federline's PoPoZao and the like. This applies to the language you hear people use. When looking back you'll forget the hundreds of times your grandmother swore in your youth, but notice the times just last week your son/daughter did.

2) Increased record of daily life - With the furthering of the internet, people have more and more record of the things that happen. Closed Circuit Cameras, blogs, Twitter, and tens of Cable News Networks make it so every little detail goes noticed. Back in the 1400's, the average housewife could swear herself silly trying to get the cow to give milk or whatever task they were doing at the time, and few would ever hear let alone jot it down.

3) Expanded definition of what is profane - With the invention of the FCC, the government had to decide what was illegal to say on TV and what wasn't. Early in the 70's you could get away with saying a few words on tv that you can't say now. Moreover, with the increase in communication there has been a surge in the multiple slang words to define the same few sex acts, or other things we have dubbed profanity.

I really don't think much of the increase has to do with a decreasing of morals in our society.