Even if it’s not dangerous, going out in heavy rain isn’t exactly the most pleasant experience. Especially if it’s a first date, going through that frustration, driving in it, getting wet, it can put a damper on the whole thing, and this is particularly important to keep in mind when you’re making a first impression. Further, a date SHOULD be fun, and going out in the rain isn’t. I think requesting to delay or wait out the weather is completely reasonable, not only on his part, but for your sake as well. Obviously, you have the right to say no, but it seems absolutely reasonable to ask.
IMO if he wanted to keep his feet dry, this would have been the moment for a white lie, or a simple “something has come up”.
So you’d rather someone lie to you and say “something came up” rather than be honest and say he’d rather not go out in the rain and try to reschedule? I’d prefer an honest reason every time. Hell, given that that’s the reason he gave, if he’s really interested and it’s the only time you are willing and able to make for him he very well may suck it up. But if he gave no reason, it could be anything from cold feet to a death in the family, and instead of thinking he’s being unreasonable for not wanting to trek out in heavy rain, you might instead think he has cold feet when he had to go to a funeral.
Frankly, it sounds like you’re doing him a favor. He made a simple and reasonable request about rescheduling and that’s enough to cancel the whole thing for you. I get it, you’re busy, many of us are busy these days and it’s difficult to make time, but if it’s important, you make time.
Speaking for myself, in general I’m busy every night of the week except one, so if I am going to go out, that’s the night I prefer. If something comes up on another night, I decide whether or not it’s more important than the other stuff I’m doing. If it is, I rearrange or cancel my other activities, if it’s not, I politely decline.
So, if it’s a date, if I want to see her, I will generally rearrange my schedule on a night I’m busy to see her. If I’m not willing to do that, then I’m obviously not interested enough in her to make meeting her a priority, and I’m doing both of us a favor by moving on.
So, yes, in my opinion, you’re being a diva by expecting way too much out of a guy you obviously have, at best, luke-warm interest in, for a first date in pouring rain.