For such a rough guy, his lips were ever so soft. Morty could marvel over them for hours - usually up close and personal of course. It was the weirdest paradox, to have Rick's rough and calloused hands in Morty's small and soft ones, and yet Rick's gentle and smooth lips against Morty's dry ones.
He would find himself thinking about Rick at the least opportune times of the day, usually at school during a lecture he didn't particularly care about, or in the middle of a conversation with Jessica.
To be fair, it didn't bother Morty much to think about Rick. At first it had, but as time went on, he adjusted to the bizarre idea of being so strongly infatuated with his grandfather. It was too convenient to always be in the same house as him, usually only feet away from him in the garage or maybe beside him at the dinner table. It was hard to think about their relationship as unusual when it was one of the few social interactions Morty experienced on a regular basis.
Rick of course hadn't noticed what was truly going through Morty's mischievous mind when his grandson would watch him work on his projects - or at least, he didn't seem to notice. It had taken a long time for Rick to come to terms with these feelings they had developed for each other, but in the end, there really was nothing too immoral for Rick Sanchez. He had been to the end of the galaxy and back again, and had seen things much much worse than the things he did with his grandson here on Earth. Truthfully, only the initial confession had phased him, after that he adjusted unnaturally quickly to the idea.
Morty still spent a significant amount of time just hanging around Rick in the garage as he worked. He found it simply captivating to watch Rick when he got seriously into something. He would lose himself in his work and start to mumble as he would tinker with some strange piece of space machinery to create a product Morty could barely begin to comprehend. Morty would let himself get lost in fantasy as he watched his Grandfather work, imagining Rick's rough hands running over his sensitive skin and pushing him down onto the bed without so much as a 'hello'. He loved to think about all the dirty names Rick would call him while they made out, calling him a horny slut and a dirty whore as his hands would drift all over Morty's body, raising goose bumps where he touched and often long red lines when he would drag his nails.
God knows how much trouble the two would be in if anyone ever found out. How would they explain that to their family? The short answer: they couldn't. Therefore, they would go to great lengths to hide their nefarious relationship, including Rick installing a sound proof force field throughout the walls of the garage and both of their rooms. They were also careful to keep an appropriate distance from each other whenever someone else was around, which proved more difficult than either had imagined.
As it turned out, Rick enjoyed teasing Morty relentlessly whenever he knew Morty had no other option than to play it cool. This included cornering Morty in the kitchen when Jerry was watching TV, and starting a seriously competitive game of tonsil-hockey while Morty tried to make dinner. Of course, Rick was acutely aware that Morty's self-restraint was well bellow the point of being able to deny Rick a kiss when things got hot and steamy. Dinner had been burnt more than once during these silly games, which just lead to Rick suggesting that Morty might need a little more "help" in the kitchen the next time, which would typically lead to another session of intense making-out on the counter while dinner once again burned in the oven beside them.
One time, while Morty was doing laundry, Rick had stumbled in - half drunk, as per usual - and had started to convince Morty that he may as well wash the clothes he was wearing while he was here... He had made a convincing argument. Rick always knew how to talk to Morty to really get him turned on. He would lean in, until his mouth was almost touching Morty's ear, and start to tell him just how incredibly sexy he looked without a shirt on. He knew how to drop his voice just a note or two until his gruff voice seemed to hit a cord inside Morty's brain, and the boy would melt right into Rick's arms.
Once Morty had removed all articles of clothing, and the washer had been started, Rick didn't waste a minute to put his mouth all over Morty's, and to let his hands wander down between Morty's legs to start getting him off before someone else decided to stray into the laundry room. Of course, someone had to ruin the moment for the two. Summer had put a few pieces of laundry up to hang dry a day or two earlier, and had chosen the least opportune time to come and pick them up. In a last-ditch attempt to save himself from being kicked out of the house for good - or worse - Rick had shoved Morty into the storage closet in the corner and 'casually' began to fold whatever laundry happened to be in the dryer at the time.
When Summer asked why he was folding Jerry's clothing, Rick had just burped in Summer's general direction and mumbled something mostly incoherent about helping Beth out around the house. He had kept himself turned away from the door to make sure she wouldn't notice his inconspicuous hard-on, and luckily, Summer didn't ask any more questions.
However, the only lesson that the two learned from this encounter, was that no one ever checks the storage closet.