Happen recently with a FWB and myself.
Took her for a long ride down to the dragons tail last year since we were both had a decent amount of PTO to use up. Her first time ever on the bike for longer then 20 minutes and about 40 minutes into the ride she taps me on the shoulder and asks me to pull over. We were on back roads at that point so hardly ever see another soul in sight. I pull over and she walked to a tree and put her helmet on the ground next to her. She then leaned against the tree like she was out of breath facing away from me. I thought something was wrong so I immediately went up to her after removing my helmet and once I was able to turn her around I saw her face and how flush her chest skin was I knew. She gets extremely flush on her chest above her breasts so between that and the sudden jaws of life kiss I figured she loved the bike ride a bit too much. Moments later she confessed that she asked me to pull over as she was getting a bit sensitive and she needed cock right then and there. Who was I to say no.