One answer to why he'd lie: the thing Mephisto needs is the strongest possible scar on Peter's soul. The more Peter believes he is giving up, the more powerful the score for Mephisto.
Another answer: he knew Peter would ultimately take the Deal and it's funnier to Mephisto to turn the screws.
Another answer: Mephisto can see the future and knows a future Parker child will be his downfall. By telling Peter taking the Deal prevents that child from existing, the echo of the Deal creates a psychic block that makes Peter not want children.