What definition of "middle class" are you using that includes "can't pay the rent/mortgage"?

Note that losses from rental properties don't count as housing costs, that's just poor business return.

We were talking about people delaying/avoiding children for financial reasons.

That doesn't imply "can't pay" necessarily, but when rent/mortgage payments are high, that leaves less spare cash floating around for the additional/unexpected expenses, a scary prospect.

Even those on comfortable incomes can find them unexpectedly disappear.

I think taneq is implying that while you might beleive you're middle class, the fact of struggling with rent, by some definition of middle class, makes you not middle class.