We've come to a point in this current era where there's only one acceptable response to a child being gay and that's "accept it " and any concerns or fears or feelings harboured by a parent are now "wrong" and that's that.
Many parents do have thoughts (that now can't be voiced without attack) that differ from the accepted speak.
It doesn't mean they're homophobic or evil.
There is a current trend of teenaged girls (especially vulnerable ones with mental health difficulties) clinging on to the latest "rebellion" such as identifying as "lesbian" when they are in fact not gay. Some may say "so, does it matter?"
But I believe a parent , especially an older one that was bought up in a different society to today's, sometimes will have concerns such as "I'm worried my son may be at risk of HIV" "I'm worried my daughter will get picked on by men " "I'm worried I'll never be a grandad"
Of course, we know all this feelings can be explored and explained away, but not everyone was bought up in the current climate and don't readily have the knowledge that others do. Some people aren't good at wording those feelings and instead it comes out as anger or fear.
I think it's worth having a sit down conversation before writing your DH off as a "homophobe" (not that you suggested that ) he'll come round if your DD does end up being gay. He loves her after all.
Underage sex may also play into his concerns which is understandable.