I desperately want to have kids and my girlfriend is on board, but she makes next to nothing and I make quite a lot of money. I tried an online calculator and if things go awry between us, I'll be paying her roughly a quarter of my post-tax income for the next two decades here in California.
That's just too big a gamble for me. So even though I desperately want children and I can afford them and everything would probably work out fine -- here I am, aging and childless while my girlfriend nears infertility because I can't justify the math to myself.