Because most LGBTQ people naturally shift to different sexualities/genders later in their childhood or teens, and most parents grew up in a non-pro-lgbtq era. They grew up in an era where you didn’t see pride parades or lgbtq parties often.
Also, because most parents witnessed their children being assigned to their sex (gender and sex is a difference, you’re stuck with your sex the entirety of your life, but gender is changeable), like elo said, they’re gonna assume your cis.
Heterosexual and cisgender people have been the norm throughout all of human history up until 20 years ago, when lgbtq people became much more frequent to see.
Also, a lot of people want biological children. Although yeah, there is a way for one lesbian mother to have a biological child (the other mother will not be biologically related to the child), gay dads gotta settle for adoption. Heterosexual relationships are the only way for both parents to be biologically related to the child. But relationships are about love, not blood, so remember that any parents can be good parents.
