I think a lot of us just grow accustomed to censoring ourselves. Since I don't really know you I'm not going to pretend it'll be some huge weight lifted off your shoulders, but if your friends and family don't know that you're gay, there are definitely times when you're hiding a part of who you are, or going out of your way to censor yourself, or just deflecting questions/comments in general. A friend's sexuality is always revealed over time unless they're actively going out of their way to avoid talking about it, and that takes effort.

It wasn't a huge relief when I came out, but I was glad that I didn't have to smile awkwardly and agree when somebody was talking about an attractive girl or deflect conversations about girls and girlfriends in general. Being out has also made it easier for me to open up and talk to my friends and family about issues I'm having. I was so guarded when I was in the closet and I didn't even realise it. Coming out really broke down those walls because I didn't have to worry about hiding anything anymore. I lost friends and also one of my parents, but I'm so much closer to everybody else and it was so worth it for me.

I don't know, I guess I just feel it's important that my friends and family know the real me, and being gay is a part of that. Being out doesn't mean that being gay defines me like you suggest in the OP, it just means that everybody knows it's part of who I am. Working out is only a small part of who I am too, but that doesn't mean I have to hide the fact that I'm going to the gym or whatever. It's not like I only have to tell my friends/family about the big things that completely define me. They can know the other things too.

I mean, did your friends know about the ex who wrote the letter? Do you know about the men/women your friends date? If one of your friends mentions that he's going to his girlfriend's place later, even if she's not the love of his life or whatever, is he suddenly defined by being straight? Of course not, and you wouldn't be either if you mentioned going to your boyfriend's/guy you were dating's house.