#interclan relationships

There's a couple reasons, yes, and they all border around the same issue: the kits themselves.

Nowadays, the code states that the mother's Clan has automatic ownership unless the queen or someone of equal authority gives them away, but it wasn't always this way. There have been a number of battles started over this kind of situation in the past before the law was put in place, and the losses on both sides were not worth a single litter. However, Clan cats being Clan cats, they were driven to claim these growing warriors as their own, and so they kept fighting. After entirely too many deaths, it was agreed to make a law to prevent further altercations, and by that point Reason 1 for No Cross-Clan Relationships was acknowledged: the kits will lead to fights for who gets to keep them.

Reason 2 is a less immediate, but inevitably obvious one. The Clans, as said once before, are sort of in a class of breeds all of their own, meaning that each Clan looks drastically different for each other and are built for entirely different lifestyles. A ShadowClan cat will not do as well in RiverClan - they'll struggle hard to adjust to the unique challenges of that territory. That goes for every other Clan in every other home. Now take one of these Clans' members, that is bred to survive in one specific area, and shove their genetics into a kitten whose mother is bred to survive in an entirely different area. The kits will struggle in some way no matter where they live. A half-WindClan apprentice will have a hard time climbing after their pure ThunderClan fellows, and likewise half-RiverClan warriors can't keep up with the rest of WindClan. It's going to be difficult, and, more importantly, it's going to be noteworthy.

Which leads us to Reason 3: the kits are not going to be able to get away from their lineage. That may not sound bad at first, but some things to consider are the current day climate and physical appearance. What if a half-Clan kit's father is from a Clan his mother's is currently fighting against, or will be fighting against by the time he's grown? And what if that kit is the spitting image of their father in a Clan that has drastically different colors and body builds (you tell me a RiverClan cat can look like a WindClan cat and I'll laugh in your face)? The Clans believe that kits are innocent and separate from their parents, but it's a lot harder to say that when your Clanmate looks a lot like your current enemy. Cats aren't perfect, and their minds will slip up on things like this.

Finally, Reason 4: loyalty conflicts. Because of everything listed above, a kit may not be comfortable living in their current Clan. They may look across the border wistfully, wondering if their life would be easier with their father. They'd hate themselves for it, because the number one rule is loyalty to your home, but it won't leave their head. But even if they left, who's to say they'd be successful, or even welcome? They grew up in another Clan, after all, and they clearly don't mind abandoning their family. So maybe they get bullied, or ignored, or they just can't figure out how to hunt and fight like the others. Worse, the leader might use them to get intel on their current rivals. No matter what, they're living with a paw in each world, even if they don't want to. It's hard for everyone, but especially them.