The same reason why you or I won’t accept marriages between brothers and sisters. (At least in most cultures)
As society progresses, our values change. We might consider inter-sibling marriages a regular thing in a few hundred years.
If you are conditioned to believe something, you treat that as the absolute right to wrong.
We think that siblings can’t have a sexual relationship/marriage and don’t see that as a good thing because our society has taught us so from the beginning.
Similarly, LGBTQ+ rights were really not a topic of discussion during our parent’s/grandparent’s times. so they are conditioned to believe that same-sex couples are not a ‘good’ thing.
However, your thought process can change. How fast that changes totally depends on the person. Some parents have started supporting and accepting LGBTQ+ as a norm, but others might take some time.
Changing people’s thought processes, significantly if the person has grown up believing something, takes time.
