The Cost of the Crest of Flames

The full extent of the power of the crest of flames hits Byleth when she receives the Sword of the Creator. The more she uses the crest of the flames, the sword, and the divine pulse, the more her emotions and desires demand her attention. Each crest has its own needs and is different with how it affects its users. Having such a powerful and rare crest, Byleth learns that its use requires a heavy and needy payment on her end or else she is left shaking and weak (equivalent exchange). Learning that its safer for her and her students to deal with its demands, Byleth finds herself in multiple sexual relationships in order to even out the cost of using her crest.

Basically the crest of flames needs sexual release or Byleth is useless.