creatures having sex in my backyard

My rhubarb leaves have lots of holes in them and I instantly blamed the hundreds of slugs and snails that have bread in the recent rains.  But no, the creatures are these horny little things (although I caught them chomping on my weeds this time):

 

I don't know what they're called, but when I first saw one, I thought it was just a weird creature that looked like it was two creatures back to back.  I didn't realise that it actually was two creatures back to back.  And it's not just this one that I've photographed, all of these creatures are constantly copulating.  Imagine eating and having sex at the same time. All the time.  How bizarre!