Caught by friend with guy

Not sure where to categorize this, I'm a straight 20 year old who has had an ongoing side relationship with a longtime friend who is gay. About a month ago a girl who is friends with both of us told him she saw us fooling around. She told him she came by my house and both our cars were there but no one answered the door. He says she looked through a dining room window that wasn't totally covered and saw us jerking each other on the couch in the den. I'm in a panic and so is he, I checked that window and it was a clear view. He's gay and says he guesses he will just come out if she tells people. I don't know what to do, I have had relations with this girl off and on for a few years, we're like ###$ buddies.
She told him she wouldn't say anything and was cool with it but I'm terrified. If she tells one person it's going to spread like wildfire through our community and all my family and friends will find out. The worst part is she actually saw me go down on him. Should I confront her and pled with her or what, I'm terrified..

Well, I guess it's too late for any advice that could have been given about my situation. My gay friends sister confronted him about what her friend had seen. He immediately came out to her and they had a very emotional moment that ended well, she told him she would not out him to anyone but wasn't sure about Carol, the girl that saw us. She said Carol only told her because they are close friends but was concerned that she would mention it to her brother and a couple of others possibly. I am officially terrified now, I saw his sister the next day, I've always had a crush on her but never acted on it. We ran into each other getting gas. The look on her face was very pleased, she was nice but she sort of chided me saying " I would have never guessed about you, my brother I have always wondered, but never imagined you" with a chuckle. I tried telling her it wasn't like that, that I had been drinking and it was a one time thing to which she replied her brother had told her everything about us from the beginning.
I just know that once word gets out about her brother or he comes out like he said he was everyone will find out that I have been doing this. It will spread like wildfire. I don't know what to do, I've hardly slept for a entire week. I'm thinking of just leaving, just taking what little I have and just run.

Come down, Jack! What is the worst thing that could happen? Face it, you are bi, otherwise you would not have had a boner, especially after drinking. And know what, that is perfectly fine, this is the 21st century! Do not be ashamed, be proud. Welcome to the family

How have things been since your last post?