They are exceedingly rare. A recent study that caused a lot of stir was from the University of Texas (I think) that looked at parenting results of straight vs. gay. The preface of the study noted how very difficult is was to find gay couples that stayed together long enough to be comparable to a straight couple in raising children.
Yes, I remember reading about that. Detractors of the study complained that the study was biased because it didn’t include a statistically significant number of children raised exclusively by gay couples with long-term relationships analogous to those of nuclear families. It didn’t seem to occur to them that perhaps the reason that study group was too small was that there are so few subjects who meet the inclusion criteria.