Only when you look at it from a modern perspective. It's the same concept as keeping the bloodline noble in real-life, and in the Naruto world, most of the noble clans try to maintain the density of their bloodlines. Although it's not stated in the show, we're fully aware that you can have blood too diluted to manifest your kekkei genkai properly. (Proof can be seen through a character in one of the books who's bloodline is diluted to the point where the Byakugan only manifests in a single eye.)
Since most noble clans can manifest their bloodlines properly, and it isn't the case that we have diluted bloodlines everywhere, it's a fair assessment to say the clans have been keeping their eyes on their genes.
You say we can't judge Naruto by modern standards. But they still have radios and TVs and some modern customs in the world. It's a....weird setting XD