It depends on creating a vivid and sensual setting. You need to describe the emotions and physical sensations in detail to make it realistic and engaging.
I'm not sure specifically what this 'taking 1st big cock story' is. It could be a very specific or even a made - up story within a particular context that I'm not familiar with.
By using detailed descriptions of their internal thoughts. The story can show how the futanari is constantly in a state of almost - there but not quite, with their mind racing with different emotions and sensations. For example, they might be thinking about their own insecurities or the expectations of their partner.
Imagine a group of tourists visiting a rural area. They decided to take a walk outside. As they walked through a field, they came across a flock of cocks. There were dozens of them. The cocks were not afraid of the tourists and continued their activities. Some were fighting over food, others were preening their feathers. The tourists were amazed by this sight of so many cocks outside and one of them wrote a story about it later, which could be an 'outside taking lots of cock story'.
Since the original topic is inappropriate, we can talk about 'Tell a story about the first encounter with a cockerel at a farm.' I visited a farm with my family. As we walked around, I saw a cute cockerel running around. It was pecking at the ground looking for food. I was so intrigued by it that I watched it for a long time.
First, quickly and quietly back out to give your roommate privacy. Then, later when things have calmed down, you could have an honest conversation about boundaries and knocking before entering rooms. This helps avoid future embarrassments for both of you.
The shock can be quite overwhelming. You might want to find a quiet place to sit and process what you just saw. Maybe talk to a friend (without going into too much detail) just to get some perspective. Give yourself some time to get used to the idea and then face your roommate when you're more composed.