This story has its genesis in real life events and real people. I've just imagined it going a bit further than it actually did. Enjoy!
I was in my mid-20's, in supreme physical condition, and in a relationship with Cass. Cass was a year younger than me, average height, brown hair, pretty face with an athletic build herself and an absolutely magnificent pair of DD tits that, at her age then, just sat there. She tended to dress plainly and conservatively but what was hidden beneath her clothes was simply incredible!
At this stage of our relationship Cass was, it must be said, sexually unadventurous. After we had gotten to know one another I had revealed some of my escapades in the past – which shocked her somewhat – and I think it wasn't until she really got to know a few new girl friends in our social circle that her mind was slowly opened to new and exciting possibilities. Hearing her friends talk of bisexual experimentation and threesomes she was becoming less shocked and more curious, to the point where she eventually started to bring these things up in our conversations as well. One night Cass had mentioned that she'd be open to the idea of, as she put it, being in the bed whilst I had sex with another woman.