I think my dad may be gay or bi. Should I tell him?

Let me start off by saying that I love my dad and would have zero problem whatsoever if he were gay. It would only break my heart that he's been living a lie for nearly 60 years. I love and support him no matter what.
My sister called me today and told me that she suspects our dad is gay. She lives with him and overheard him talking on the phone in a tone she'd never heard before; super happy and sweet (my dad, while a good person, is usually pretty stoic). She said she overheard him talking about how the person on the phone never would've talked to him if he hadn't fibbed about his age on the website. When she walked past his open door, she said she could have sworn the voice on the other end was male. The next day she said he was unusually happy, inviting her to hang out with him in the kitchen and talk about his day (again, while he's a loving father, he's usually not one for small talk). Mid way through their conversation he received a phone call, and said "Sorry, I have to take this" in a tone that implied she should leave the room. She said he conducted the conversation in hushed tones, but she swears she heard him say, "I've done it before, but I wasn't in love with the guy." There was also an incident at a family wedding over Thanksgiving where our aunt's boyfriend got into a huge fight with one of my uncles at a family dinner, and before storming out of the restaurant he yelled, "By the way [insert my dad's name here] IS GAY." At the time we all laughed it off as a bizarre and outlandish comment, but now it seems very relevant. Also, his birthday is June 26, and when I asked him what he was doing he said he was going out to Hollywood to hang out with his openly gay friend that he's had since before I was born, which I thought was odd because obviously Hollywood would be mayhem the day gay marriage was legalized, and he usually hates crowds. And obviously his friend would be out celebrating. I didn't think too much about it beyond a "that's weird" at the time, but now it also seems like it could be a piece of the puzzle.
It's confusing because ever since my parents divorced when I was five, my dad has dated women constantly. I guess you could call him a "womanizer," except that every girl he dated, he dated with intention. He was just constantly dating. He's been engaged twice since the divorce, but never remarried. It never once crossed my mind that he might be gay. I think it's more likely that he's bi.
My siblings and I have discussed it and have differing opinions on how to approach the situation. We all just want him to know that we love him and he has our support. My sister wants to sit him down, intervention style, and tell him we know, but she wants to wait until he's had more time to figure things out for himself, since judging by the phone conversations it's a fairly new adventure for him. My brother agrees with me in that an intervention style meeting might come off as an ambush, and thinks that my sister should talk to him by himself and let him know that we all know.
I just really want to talk to my dad myself. We've always been close and it kills me to keep a secret from him, even when that secret is that I know the secret he's keeping from us. I want to tell him that I love him and support him 100 percent and that I'm sorry he's had to hide, and I want to tell him as soon as possible. It just feels wrong to me to pretend like nothings going on, and I'm afraid he might think he can't tell us or that we wouldn't be accepting (he knows we're supportive of gay marriage, but maybe he feels like this is different because he's our dad?) , when maybe knowing that we know and are okay with it could be liberating for him, and he might be a lot happier knowing that he has our support. My sister, however, has made it very clear that if I tell him I know before she's ready, she'll never speak to me again.
So, my question is this: What would be the best way to approach this situation? Should we tell him that we know, or wait for him to come to us? Any advice is greatly appreciated.