The saiyans have the ability to massively boost their chi and therefore their defenses. But assuming they are not circulating their chi, they actually have comparable defenses to a base human, hence why they used to wear armor when engaging in battles. If you get caught off guard, even a significantly weaker opponent can injure you.

Thing is, saiyans on earth have learned to get along and "open up their hearts" aka make themselves vulnerable. Little more than headcanon, but it seems they do not keep their chi circulating when around their wives, hence why you can have sex with a saiyan without getting your delicates kamehameha'd