In the first canto of Byron's Don Juan, we are introduced to his first lover, Donna Julia. She is married to a fifty-year-old man by the name of Don Alfonso, and yet it's clear that she's completely unsatisfied with her marriage. Because of this, the speaker thinks it would've been better for her to have married two twenty-five-year-old men instead.

The implication here is that Donna Julia is a promiscuous young lady who is chronically incapable of settling down with a man for any length of time. That would explain why she sets her sights on Don Juan. Julia sees her affair with Juan as a means of escape from a marriage that has brought her nothing but unhappiness. As she tells us herself,

It was for this that I became a bride!

For this in silence I have suffered long

A husband like Alfonso by my side.

In this particular part of the poem, Byron trades on the stereotype of the Latin lover, a fiery and passionate being given to indulge their sensuous desires at the drop of a hat. However, the poet doesn't blame Don Juan and Donna Julia for their behavior as they embark upon an intense love affair; he puts it down to the warm climate of the land in which they live:

'Tis a sad thing, I cannot choose but say,
And all the fault of that indecent sun,
Who cannot leave alone our helpless clay,
But will keep baking, broiling, burning on,
That howsoever people fast and pray
The flesh is frail, and so the soul undone.
What men call gallantry, and gods adultery,
Is much more common where the climate's sultry. (canto 63)

Apparently, hot weather does certain things to people. It makes them want to have passionate love affairs, even if they're already married. That's what Donna Julia does, a woman who should never really have chosen to marry a man of fifty in the first place.