Steve and Tony end up stranded alone together on a tropical island (or it could be the savage island). They have to work together to find water and food, build shelters, fight off wild animals and the cold at night by sharing body heat (because of the sharp drop of the temperature) all that kind of stuff. They need one another to survive.

This is can be full-of-angst post CW, or somewhere in MCU or Ultimates, etc.

That develops in them ending to have sex.

Bonus points for them thinking it's just about sex, then them both realize they're in love with the other and start pining for the other. Happy ending would be lovely though.