Should I be kicking myself?

Some oil coming out of the breather when you kick it shouldn't hurt, I'd think.

... many and various answers to your tap query. I had a Yamaha non-return valve on my ES2 and it worked very well - it was an update on the Velo type fitted to all spring frame big singles.

As to whether you've done any damage I doubt it but I would drain the crankcase via the hexagon drain plug.

I have seen people switch taps off even when they stop for a coffee or petrol fill up - totally unnecessary as unless the oil pump is completely worn out only a few ccs will drain down in a short stop. A week or more - yes, possibly. There are a lot of opinions here

My Atlas did this a few times, made a mess but no lasting damage.

Interesting,  when I bought the bike it would not wet sump at all until I washed the oil tank out (had at least an inch of sludge in the bottom) and changed to multigrade.
It had SAE 30 in it according to the previous owner.

Johnny Doig suggested that I drain the oil tank into a container at the end of a ride and leave the container beside the kick start to remind me to put it back before the next ride.
Very good advise and my go to method to avoiding wet sumping.

Don Anson
Melbourne 

 I think draining the oil tank is easier and less messy than draining the crankcase but obviously needs to be done before the bike is left standing.