Have you ever read or watched a story in which characters are falling in love and succumbing to lust during situations that call for their attention to be elsewhere?
It can seem unlikely from a practical angle, but I think the reason we so often relate to love during a disaster is the comfort it offers. The characters (and readers) get a break from the gravity of loss, death, and destruction swirling around them.
In many cases (as happens to Irina) the seeds of love are planted before the plot goes sideways taking a sharp turn into catastrophe. How does it go down? You'll have to read the books to find out.