I'm not suggesting that females fear all males, nor that we should. But your reference to dogs is a good one. Some dogs are dangerous, most are not. Lots of people have been seriously injured by pit bulls. On the other hand, every pitty I've ever known has been an absolute sweetheart and gentle as you could imagine.
When I encounter a strange pitbull with whom I haven't interacted before, I'm going to be on guard - just as you note. Not necessarily fearful, but aware that this unknown dog has the potential to injure me, to do me harm. If you were to lock me in a room with an unknown dog, my level of guardedness is going to be escalated because not only is this an animal that could potentially injure me, but because my ability to get away from them is constrained.
Most females aren't constantly walking around in a state of fear about every male. In the majority of situations, we don't have any fear at all, and our guard level is minimal. When I'm at work, or at a crowded store, or out with friends, my guard level is pretty low. When I'm out with my spouse, my guard level is as about as low as it's possible to get (they're a big enough, tough enough, trained enough person that nobody ◊◊◊◊◊ with them either).
In some situations, my guard level is going to be a lot higher because of the nature of the constraints. If I'm alone in an unfamiliar part of town, or late at night, for example. Walking through a dark parking lot. And perhaps counter-intuitively to you, when I'm in very crowded venues like clubs or concerts or subway cars.
Sometimes, its the actual person that raises my guard level higher. I'm sure you've run across it, sometimes someone just gives off a creepy vibe - there's just something that gets your neck hairs up, that speaks to your inner caveman and says "danger". It might be an incorrect assessment, it might be a poor judgement, but it's still there, it's a real thing.
I assert that it's reasonable for people in general to have a variable guard level depending on the people they're interacting with and the venue in which they're interacting - the degree of perceived risk and the constraints placed on our ability to mitigate those risks all play into our internal defcon levels.
I'll further assert that in general, it's reasonable for females to have a higher defcon level around males than around females, all else being equal.