The fuel doesn't ignite at "low" temperatures, one reason is because the whole airplane leaks fuel until it's at an high enough speed. At that point, maybe because of the surface temperature, or the forces it tackles at Mach 3+, the whole thing becomes airtight.
Yup, at Mach 3+ the fuselage expands so significantly that gaps need to exist when it’s stationary, otherwise it’d buckle. So the gaps mean it leaks fuel when stationary.
At least, that's what I remember.