When Gradient was born, Lust was about 21. When Lust met Gradient, he immediately fell in love (not romantically yet) with the bean.
Lust was like Gradient's gay uncle...at first.
The two would play around, Lust would spoil the shit outta him when they shopped, and would sometimes cuddle together to help Gradient sleep.
Gradient suffers from horrible nightmares so as Lust stayed with him while he slept, it comforted him.
Ink had slight worriness, thinking Lust would do something to Gradient but Error made sure nothing bad would happen and nothing did.
As time went by, Gradient got older. At the age of 16, he started to see Lust not as his uncle anymore but something more. He asks his dad Error what he felt and Error told him.
Gradient was worried, thinking Lust would be disgusted so he tried to hide it.
When Gradient became 18, Lust had a very special present for him. Lust was now 39. He gave a giant heart pillow to Gradient and lots of chocolate and a small card.
Gradient opens the card to see it was a love confession from Lust. He finds out Lust started loving him when he was 15 but knew he should wait till Gradient was at a legal age to tell him.
Obviously, Gradient accepted his love and they then become lovers. Ink obviously doesn't like it because of the 18 year age gap but Error doesn't really mind, as long Lust doesn't hurt Gradient.
The end :D