Markdown seems pretty unanimous about how to treat that 
and we are doing the right thing.
If you want a monospace font in your editor go for that, by adding a site customization.
I am torn, my personal pref is for monospace markdown textareas, but there is a strong argument monospace makes this “more computery” and “less friendly”. Ultimately this is a decision that is up to @codinghorror. Even cooking Stack Exchange uses monospace (but on the other hand they do not display stuff side by side).
----A----x----B----x----x----HEAD
\
x----x----C
