To be honest, and in my experience it is mainly a youth thing.
When you are growing up you a need to be defined, and you define yourself by how you define your friends, and differences and similarities are important.
I find that when you become and adult, and move in adult circles it does not really matter, and nobody cares whether you are gay or straight, or a vegetarian, or Muslim or Jewish. Not so much. Who cares if John in accounts is sleeping with Selma or Peter. Or if Jane is in love with Peter or Selma. People are defined by so much more than their sexuality. The books they read, the profession they are in, whether they are a parent or not, etc. As an adult, it is not so much about "coming out", it is just a question of how you refer to a partner.
Like the other day at a dinner, my friend John was talking about his sister Linda. And as part of the conversation, he said, Linda and her partner Sue.
Or I was on a plane recently, striking up a really interesting conversation with the woman next to me, and she was referring to her partner and her partners son, and how hard it was to up and move when you had a child in secondary. Off the plane, she introduced me to her partner, with a big bright smile, "This is my partner Eva".
So, it is an interesting question, why the need to "come out". Why the need to put a name to your differences and similarities?
I think these days, unless you are young and need to open up about this part of you to your friends and parents, it is not so much coming out, but mentioning the gender of your partner in passing.
I have seen very few examples of homophobia, maybe because I grew up with many gay friends, male and female, and also know people who have done sex changes. To me, the whole mix of sexualities was the norm, rather than heterosexuality.