We know Mephisto is a softy at heart. He loved Mary Jane and Peter Parker's marriage so much he had to take it away for himself. He also likes to torture and make people suffer. Since both Darkstar and Fantomex are both dead and in HELL, will Mephisto do the ultimate douchebaggery and marry them for all eternity? What better way is there to make them both suffer? Forcing him to spend eternity with a woman that is the opposite of Psylocke and making her the sexual slave of one of her many killers? It's the perfect suffering for both! Suffering through the power of LOVE. Mephisto truly is evil.
*Crappy Photoshop because there aren't enough images of Darkstar to make something better