I think the closer you are to the shared root meaning (Spanish and English) the more likely you are to have your answer accepted. As you stray further from that shared root, the number of possible translations grows. As we know context allows you to shade your meaning with other related words. So, I would suggest accepting the DL translation and as your confidence and vocabulary grows feel free to venture off with alternate translations.
I’m not convinced this is all there is to it. From what I’ve read, él can be used to mean “it” when it is the object of a preposition. So given appropriate context, couldn’t this sentence indeed mean “I am against it”? For example, wouldn’t the following be correct?
Mi hermana tiene un plan. Estoy en contra de él.
My sister has a plan. I am against it.
(Here él refers to plan, which is masculine.)
From what I understand, the "en" and "de" aren't necessarily required, but it's a common expression, especially when giving emotional emphasis. Just one of those things to learn. (More about "contra": http://spanish.about.com/od/prepositions/qt/Contra.htm)
The reason the words aren't combined is that it's not "el" (no accent: the), it's "él" (with an accent: he)—two different words. "De" and "el" (no accent) combine to mean "of the". "De" and "él" (accent) do not combine.
I saw the tip here that estoy is for a more temporary condition and soy is more a permanent condition. Could we say "soy en contra de él" meaning something like "I am against him [and always have been and always will be] " - a more extreme disposition. Does Spanish allow you to be creative like that?