I guess it was by accident originally as realise that the seeds were first sown about 6 months into our relationship after I watched Natasha being fucked by another guy. Long story, but we'd gone out as foursome with another couple and inevitably had too much to *******. He was an older guy with a girlfriend about the same age as us. We ended up back at their place, watched some porno films which resulted in the other couple having sex in the room while we watched them totally transfixed.
At some point the girl indicated for my girlfriend to join in. After just a moment hesitation I kind of nodded my agreement and so Natasha went over and sat on the guy's cock while the girl lifted her off her dress off and removed her bra. Being already so turned on sitting astride him Natasha came very quickly and I expected him to as well but he had other ideas. He positioned her on sofa, finally pulling over her panties and started to fuck her again really hard.
The other girl came over to me, but I was mesmerized and more interested in watching Natasha getting fucked. Very confusing emotions, but I was so turned on. What made it all the more intense was that the guy just finished inside her without asking, when we normally used condoms!
Natasha was very ashamed after wards blaming the drinking, so we kind of buried it and it was not spoken of, though I never forgot the excitement and it took me about 10 years to suggest she again experience other cocks.