Learn Italian: Perché divorziate? Non è un matrimonio felice. - Why are you divorcing? It is not a happy marriage.

Good catch! While 'to get married' uses a reciprocal pronoun in Italian (sposarsi), 'to divorce' is just a normal verb (divorziare). You don't use a reflexive pronoun here, so it is simply divorziate, not vi divorziate.

Who exactly is the subject of divorziate?

The subject is voi (plural 'you'). The -ate ending tells us it is the present tense 'you all' form of the regular verb divorziare.

Does matrimonio mean the wedding ceremony or the marriage itself?

It means both! Matrimonio is the everyday Italian word used for the actual wedding day as well as the ongoing marriage.

Why does felice end in '-e' if matrimonio is a masculine noun ending in '-o'?

Felice belongs to a group of adjectives that always end in -e in the singular. They don't change to '-o' or '-a', so they use the exact same -e ending for both masculine and feminine words. To make it plural, it changes to felici.