Propietary tags splitted

I use on my system a bunch of propietary xml tags as in:
<myown:tag property1="one" another="2" />
but after switching back from code to WYSIWYG to code edit i get "corrected" my tag to:
<myown:tag property1="one" another="2"></myown:tag>

When I have more text ahead, the closing tag is sended to the next \n, i.e

<myown:tag />A long text of several paragraphs...and the end.

becomes:
<myown:tag>A long text of several paragraphs...and the end.</myown:tag>

wich is not exactly the same.
I presume I can correct this behavior to make it treat my own tags in the same way the <img /> tag, wich is not splitted at all. Can you ppl point me into the right direction where I can edit this behaviors?

Thank you