The whole tradition started out as a pride thing. Shiro has always been bigger, stronger, and most importantly taller than Keith. Most of the time, he doesn't mind. Shiro being an Alpha's Alpha is a rather attractive trait to his gay ass, but it poses some unique logistical problems.

If Shiro wants to scent mark him, all he has to do is literally pick Keith up on his way out of the shower and rub all over him until he might as well be carrying around a sign saying "Property of Takashi Shirogane" on his neck. That's fine, he's quite happy with that sign. The issue is when he wants to return the favor. Scent marking Shiro, especially around the head and neck, requires either an impromptu climbing expedition or the other alpha's cooperation, and having to ask someone to lean over and stand still when you want to claim them is a real hit to the ego.

Keith's solution, that did not involve having to ask for help in marking his own damn boyfriend, had been to cover the clothes in Shiro's drawers and closet with his scent. All of them, socks, pants, underwear, and especially the collars on his shirts. Didn't matter how much earlier than Keith Shiro woke up and showered. When he left the house, no one could mistake the fact that he was taken.

Shiro had been surprised, but like the good soul he was, he took it in stride. Well except for the jab about if Keith was going to go through all his clothes anyways could he at least clean them while he was at it. He'd been shocked when Keith had started doing just that. Really though, he had only been being practical. It was tough to figure out which clothes he'd already marked, when they were all in the closet. Much easier to clean them, roll around a bit in the pile then put them up himself.

The arrangement had come in handy when Shiro had left for Kerberos. The older alpha was so used to leaving the laundry thing up to Keith, he hadn't even noticed that only about half of each load was making it to the laundry machine and then back into his closet. By the time Shiro's rocket broke out of the atmosphere, Keith had a nice pile of Shiro scented cloth to line his mattress with in the place of the other man.

When they announced the pilot error on TV, the clothes had barely smelled like him any more. Keith still took them with him to the shack when he was thrown out.

When Shiro crashed back down to earth and they found themselves whisked away to pilot giant magical space lions in a war against an evil empire, both of them combined hadn't enough clothes to count as a full load. Keith's pride had also become worth so little, that he might have just climbed Shiro like a tree and gone right past the scent equivalent of "Property of Keith" to the point of "Keith's Bitch" instead. So sue him, his mate had just come back from the dead. Going overboard in the marking department was justified.

Still, when Coran had showed them how to make the futuristic cleaning system work for their clothes, Keith had dragged the freshly laundered vest out of Shiro's hand and made sure his smell would never leave the other alpha again.