Why on Earth, or even Cybertron, Megatron of all beings is falling for this odd, blue earthling is beyond his comprehension at this point.
Mystique and Megatron met when he was smashing through the city, she happened to be inside a building as he ran through it. He'd immediately rescued her from a death, and since then has refused to travel without.
The other Decepticons make much fun of the odd pair, especially Starscream. In fact, one day, Starscream admitted his jealousy of Mystique when he also proclaimed his gay love for Megatron.
After that dispute, Megatron begged Mistique not to leave him, and claimed true love for the odd earthling. The very next day, he forced a citizen to have them married by law and now live happily as husband and wife.