I haven't checked but I don't think they make a hallmark card that says what I've been trying to figure out how to say. I don't have the right words yet, but if I did, it would read something like this...
I want you to know that I'm sorry - I know you're not mad at me but I believe I owe you an apology. While I never meant to hurt your feelings or tell you who to be, my implicit heterosexual bias led me to make jokes and comments to you about looking for a "girlfriend" and so on, over the past couple of years. I know that you don't hold it against me in anyway, and yet, I am aware of how callous or ignorant those remarks were. If any of these remarks ever hurt or offended you, I want you to know that I am truly sorry. And even if they didn't hurt or offend you, I still feel responsible for unknowingly putting you in a box. If heterosexual orientation was not a matter that we took for granted, no one would ever have to come out of the closet, or, put another way, we would all have to take the time and effort to discuss our true sexual orientation, so each and every person would have to come out of the closet and acknowledge who they are. And then, their friends and families could stand up and cheer them for the courage it takes to discover who you are. I truly applaud you for being who you are.
As I think back, I could easily make the excuse that I didn't know, but that's not a good excuse as far as I'm concerned. Unknowingly, I took you and your sexual orientation to be a certain way, and in doing so, imposed an arbitrary heterosexual bias upon you. In this way, I didn't really make room to see who you are. If I had been paying attention, I would have known all along and you would never have had to announce to me who you are. We never make heterosexual teenagers announce their sexual preference, and yet, we constantly expect that gay youth will "come out" to us. How ridiculous is this double standard! For my part, I'm sorry I made you participate in this double standard by having to come out to me.
I don't feel horrible about myself or guilty, and I'm not beating myself up about all of this - but I do want you to know that I acknowledge that unknowingly, unthinkingly, unconsciously, I participated in society's and out family's heterosexual bias. And I also promise you that I will support you in whatever way you think it appropriate, so that we can help our family be more inclusive.
I think you are an incredibly beautiful human being and I am so proud to call you my brother and my friend. And I truly love you for who you are.