[Avodah] Two kinds of humros

[David Riceman]

I'm studying Be'er HaGolah, and it reminds me of an old question.  Think 
of the gzeirah of einah bas yomah atu bas yomah as a typical humrah.  
The cook is in the kitchen every day grabbing at pots.  One evening, 
cooking meat, he accidentally grabs the pot he used that very morning to 
cook dairy.  So the Rabbis required him to have separate dairy and meat 
pots to avoid this problem.

How does this translate to shniyos ba'arayos? Certainly we can't imagine 
that one of the newlyweds just grabbed the wrong spouse on the way to 
the wedding! Were the Rabbis worried that no one in town knows the 
halachos? If they don't know Biblical law how could the Rabbis expect 
them to know rabbinic law? So what's the function of the gzeirah of shniyos?

The Maharal's answer is that shniyos are inherently immoral, but less 
immoral that Biblical prohibitions, so the Torah left it to the Rabbis 
to prohibit them.  But according to the normal explanation, that this 
somehow prevents violations of Biblical law, how does the decree work?

David Riceman