How can I get a plain ampersand in a post without having it get the & amp ;?

I tried to link to a YouTube video using an &start=28 at the end of the URL, so that visitors can avoid the first 28 seconds of this video, but the WordPress editor (in HTML mode), inserts the HTML entity for the ampersand rather than a clean ampersand (&).

How can I get a regular ampersand in the URL?

What I want:
https://www.youtube.com/watch?v=TGEF1Afuu5s&start=28
What I get: (in WordPress MU 2.7)
https://www.youtube.com/watch?v=TGEF1Afuu5s&start=28
What this forums gets (on March 26, 2009):
(I can't display this here, but it looks something like this:)
& # 3 8 ; start=28
(I added spaces to get it to display.)

I am trying to use it with the SmartYouTube plugin.
But it doesn’t seem to work with regular URLs either.

Any suggestions would be greatly appreciated.