My wife insists that I suck her bull's cock. I am bi, and I enjoy sucking cock, so it works for all of us.
We started in college. I was sharing her with my best buddy. He was better at sex and bigger than me, so I just let nature takes it's course. He was coming over to our apartment more and more to fuck my wife (fiancée at the time).
One afternoon, while I was watching her (up close) suck his dick, she pointed his dick towards me and dared me to take a taste. I looked up at my buddy and he said something like "I don't care. Go for it if you want." So, I did. While I did, she was grinning from ear to ear and then said "good boy." Later that night, after he left, she told me that it really excited her to see me suck his dick and she would like me to do that whenever he comes over. Also, the next time, she wanted me to let him cum in my mouth and I was required to swallow every drop.
This was added to our sex play with my buddy and it really strengthened the bond between the three of us. He ended up moving in with us. We squeezed a king size bed into our apartment and the three of us shared it.
That experience set the path for the lifestyle we still enjoy to this day.