I did a quick search and found a study about percentages of couples raising children . They have a table by couple type and marital status. The difference isn't as stark as you might think.
Looking at the smallest difference in the table, married female/female partnerships have a 30.2% chance of currently raising children, while married male/female partnerships have a 38.7% chance. Is it obvious to you that this would be so close? Would you be surprised if there were countries where the numbers are reversed?
It is obviously important for population and demographic research to know just how age, type of couple, divorce, income, etc. effects family size with actual estimates.
Your post just did a study on that topic. Should you get funded for that?