Because the choice is either to hide important aspects of yourself, or else come out, whether that's explicitly by saying something, or implicitly by your actions.
In my case, if I didn't explicitly come out as nonbinary / trans, the change in my wardrobe and appearance was going to "out" myself regardless. It seemed healthier and emotionally safer to be upfront about it with the people I know.
In your example, a person doesn't have to "come out" as heterosexual, they can just introduce their spouse, or mention who they are dating. Maybe in an ideal world that would be the case for everyone, but it's not today.
Because the choice is either to hide important aspects of yourself, or else come out, whether that's explicitly by saying something, or implicitly by your actions.
In my case, if I didn't explicitly come out as nonbinary / trans, the change in my wardrobe and appearance was going to "out" myself regardless. It seemed healthier and emotionally safer to be upfront about it with the people I know.
In your example, a person doesn't have to "come out" as heterosexual, they can just introduce their spouse, or mention who they are dating. Maybe in an ideal world that would be the case for everyone, but it's not today.