My (25F) dad (59M) and his boyfriend (27M) are in a really healthy relationship. My dad is the happiest I've ever seen him. He finally found a man who loves and cherishes him without taking advantage of how nice and generous he is. My dad and I are very open with each other, and I heard about this guy after the first date. He told me his age and that he is trans. I was so shocked by the age gap I completely blew over the fact that my dad basically just outed this man to me (the age gap is still something I struggle with accepting). He found out later that he really shouldn't have done that, since his now boyfriend doesn't want anyone to know.
Now here comes the drama. This happened months ago but it still bothers me. I made the mistake first. Because my sister (29F) is trans I made the incorrect assumption that my dad told her about his trans bf as well. I accidentally outed him to her. She didn't care, obviously, but I felt awful, even more so when he found out that my sister knows as well. He thought that she figured it out, that she could tell he was trans. I was quick to want to correct that, and admit my wrongdoing, but my dad didn't want me to. He said that he didn't want his bf to resent me for outing him. But I argued that I would prefer that to him feeling like he wasn't fully passing. My dad got very distressed. I think he's scared of losing such a good thing. Or that he needs his bf and me to get along because he hangs out with us both a lot. I feel awful that my slip up caused so much pain and deceit. I wanted to tell the truth, apologize. I hate lying, I hate that my dad lied to his boyfriend and now I can't come clean without exposing that he lied.
Am I the buttface for not coming clean, against my dad's wishes?