It's happened to me a couple of times, though with different results.
A straight friend of mine used to brag once in a while about his dick. On my birthday many years ago, I was a bit drunk and told him sending me pics would be a great present. Right then and there, with his wife in the room with him, he dropped his pants, snapped a couple photos, and emailed them to me. He had a Coke bottle on hand for comparison. We've always joked a bit about it, but it never effected our friendship.
Another time involved a male stripper at a gay bar in Cleveland, Ohio. A friend of mine talked me in to going out by mentioning a stripper there with 12 inches. By the time I made it there, he was dressed and drinking at the bar. (There'd been a bit of a fight after someone tried to take back their tip for some reason.) I sat next to him, got some liquid courage in me, and started casual conversation. He said he was straight, and I respected that. When we both were sufficiently drink, I asked him about his size and gave him a bit of grief since I'd never seen one that was a real 12 inches before, so he led me to the bathroom and unzipped. He was maybe 6" soft and said he was a grower.
Maybe it was the booze, but I got a bit bold for once. I kept telling him that I bet I could deepthroat him. He said no one ever had before, and after about half an hour of my drunken bragging, he told me he'd let me try to prove it. We went out to my car and I gave him head. When he was fully hard, I'd say he was about 11.5" but not very thick. It took me several tries (and several breaks to hurl from my triggered gag reflex mixed with way too many mixed drinks), but I managed to deepthroat him a few times. Maybe it made him super horny or maybe he saw how much I was gagging and struggling, but he told me to get on my stomach. I put my seat back as far as I could; he climbed on top of me and fucked me deep. It didn't take him long to cum. I dropped him off at home and he gave me his number, saying we should "hang out some time", but I never had the nerve to call him. (Apparently, he had a girlfriend.) For the next several hours, I still felt like he was all the way inside me and all I could get out of my mouth was "Oh my god..." Never had anyone that long inside me since then, but it's a feeling I'll never forget and will always want to experience again, gay, bi, or straight.