There’s way more to it than that - otherwise why not leave gay men alone?
Because they’re going properly Old Testament. If you think that the falling birth rate is a punishment from God for deviating from his ways, then the way to please him would be to go old school. No tolerance of homosexuality, no masturbation etc.
Serena and the Commander were madly religious, quoting the Old Testament at each other while they had sex. It must have destroyed Serena not to have got pregnant while being so devout.
When they had that dinner in the first series where they brought out the children of Gilead - that was validation that they were doing the right thing - God is blessing them with children. I expect that was also validation for Aunt Lydia when finding out June was pregnant.
If you want to read the bit of the Bible about handmaids giving birth it starts here: www.biblegateway.com/passage/?search=Genesis+30&version=NIV
Main bit “When Rachel saw that she was not bearing Jacob any children, she became jealous of her sister. So she said to Jacob, “Give me children, or I’ll die!”
2 Jacob became angry with her and said, “ Am I in the place of God, who has kept you from having children ?”
3 Then she said, “Here is Bilhah, my servant. Sleep with her so that she can bear children for me and I too can build a family through her.”
4 So she gave him her servant Bilhah as a wife. Jacob slept with her, 5 and she became pregnant and bore him a son. 6 Then Rachel said, “ God has vindicated me; he has listened to my plea and given me a son .” Because of this she named him Dan.[a]”
You’ll most likely be familiar with this particular bit of the Bible - Jacob has loads of kids with his wives and their handmaids, one of whom is Joseph and his Amazing Technicolor Dreamcoat.