Wow, I'm really honored that you'd ask me to help you with this. Thank you for thinking that I could:-) I'm not an expert by any means, and I wouldn't want to steer you in the wrong direction. But I can offer my opinion of what I'd do.
The good thing is, you've gone through this with your mom already. And no matter how she views it, she might be your best way of getting it across to your dad. Have you asked her to tell him? I know it might seem like a cop out by not telling him yourself, but your mom and your dad are equals, and she might be OK with bearing some of the heat if he's upset, or explaining it more if he's confused/curious...or just talking about it if he's not surprised.
I kinda feel like, unless your parents dont talk to each other at all and would get into a big fight about it, that your mom should help you out. I think she might even be happy that you're asking her to help you. Most moms live to help their children and to be appreciated by them. So if you tell her that you've been struggling with how to tell him for a few months, and that you really need for him to know...and that you think she's the best person to tell him, she might like that.
If your mom is not an option for whatever reason, maybe you can email him or write him a handwritten letter. You have to be prepared for a bad reaction, because we never really know how any one person will react to this type of news. But a lot of times, parents will surprise you (in a good way). So talk to him like an adult with maturity and be direct, but also let him know that you're still the same daughter he's always known...and that even if initially it is a shock to the system for him, it won't always be that way.
The most important thing is to let him know that you're happier being yourself than living a lie. And that he's allowed to ask you questions if he wants. Or he's allowed not to mention it if he wants. Give him some options...makes it easier to handle news like this.
Yeah so without knowing your family AT ALL, or knowing you very well, I can't give specific advice, but I do wish you luck...and just know that you're not the first and certainly wont be the last to go through this:-)
I'd love to hear some stories about how you came out to your parents. How old were you? What did you say? How did they take it? Did they already have an idea? How are things with them now? Your answers might help someone lurking in the shadows of this blog who needs help but is too shy to ask...so leave a comment. Remember, you can comment anonymously on my blog:-)