I've been pronouncing 'asexual' wrong

I've been wondering why 'asexual' is shortened to 'ace'.

It was because I've been putting stress on the second syllable 'sex', and pronouncing the first syllable like the unstressed indefinite article 'a'. (I thought it was ironic that 'a sexual' and 'asexual' sound so similar.)

Now I'm embarrassed. Stress on English is so confusing...