I do not understand why people care about others' sexual preferences
I'm pretty sure you know ;)
The main reason is, I think, that when someone accept other way of sexuality, other way of gender feelings and the fact that sexuality and gender are fluid, it tends to stop to consider that his way of living his sexuality and his feelings about gender is a standard or a norm.
And a lot of people, especially religious, can't accept that fact! They consider their are normal, in conformity. At the image of God (who is not heterosexual for one penny, anyway, but, they are to narrow-minded to understand that)