Here's a spin on the "keeping warm in the cold" trope.

I think it would be hilarious and hot if Jyn and Cassian were on some kind of surveillance mission and nearly get caught. They end up waiting in a small space, practically on top of each other and Cassian gets a boner. Jyn probably can't help teasing him, intentionally or not.

Why does he have an erection? Maybe they end up observing something arousing from their target. Or maybe Jyn is so alluring to him that close proximity was always going to be trouble.