Genymotion overheating MacBook Pro with Yosemite
If an application uses only one core intensively instead of balancing the load to all cores, Intel's Turbo Boost kicks in. This causes the CPU to be overclocked to boost the performance of a single core (or even a few cores). This causes increased current and power consumption, and of course heat.

The heat can be avoided by dividing the same load to separate cores (but Genymotion doesn't support this) or by disabling Turbo Boost (for example, Turbo Boost Switcher, not personally tested).