My GF had hinted that she thought it would be hot to have me under her 69 while a guy was fucking her doggie, but we had never tried it. At the time I was too straight to even consider it.
One evening we were at a swing club in Las Vegas and hooked up with a guy that she felt seemed very nice. We got into the hot tub. He and I kissed and sucked her, getting her very hot. After a shower together, we went to a private room and started to play. After we got her hot again with foreplay, I let him, our guest, go first in fucking her. He was fucking her missionary and she seemed to be enjoying it. I whispered in her ear and asked her if she was interested in me getting under 69 and him fucking her doggie. She did not even take time to say, "Yes," but flipped over on top of me in a microsecond, and started sucking my cock while I started sucking her clit.
The guy's condom covered cock slipped over my nose and slid into her pussy. As he started pounding her pussy his balls bounced back and forth over my nose as his cock went in and out of her pussy. It was difficult to keep my lips sucking her clit, as it kept being pulled up as his cock hit bottom. But, with effort I was able to either suck it or keep my tongue on it most of the time.
After a bit, his cock slipped out and rubbed across my lips and mouth. Then he lifted it up and slid back into her pussy. For the first time in my life I thought about sucking a cock. I knew the next time it came out, if I stuck out my tongue, his cock would slide into my mouth before he could pull back. But, he kept using deep thrusts to keep his cock in her pussy and did not let it slip out again.
After about 30 minutes I could tell that my GF was getting tired of being fucked, but he would not cum. I asked him if he would like for me to pull on his balls to help him cum, but he said, "No." My GF then told him that she was getting sore and had to stop. She pulled off his condom and stroked his cock until he came.
Since then we have done the 69 and doggie a number of times, and it is one of my GF's favorite positions. The next time we did it the guy was bi, and the first time his cock came out of her pussy, my tongue went up, and it slid deep into my throat. After his cock was back in her, she asked what happened. He told her and she asked me if I liked it. I told her, "Yes," as it was a real thrill. That was the turning point in me becoming bi.
Since then we have found that a lot of couples like 69 and doggie.