Romantic delusions can be expensive:

41% of [US] babies were born to unmarried moms in 2008. … More than half of the unmarried parents were living together at the time their child was born and 30% of them were romantically involved (but living apart). Most of those unwed mothers said their chances of marrying the baby’s father were 50% or greater, but after five years, only 16% of them had done so and only about 20% of the couples were still cohabiting.

These delusions seem obviously functional – people are who more confident in their partners are more attractive as partners.  But the cost of such costly signals can be great.