I took my dog over to a friend who has an undesexed 2.5yo male JRT x. (They only just adopted him and he's getting the snip next month.) They had a great time zooming around the backyard together. It actually took longer than I had expected before the JRT tried to mount Banjo. He's fairly big for a JRT x, but way smaller than my dog. So he just humped her back leg.
Most female dogs I know will tell a male dog off in no uncertain terms if they try to mount them. But Banjo seemed to have no idea what was going on and didn't get annoyed with him at all. Like with most things in life, she seemed to think it was some exiting new game. And I think she decided she must try this strange new game.
So every time the JRT had his 4 feet back on the ground, she walked up behind him and tried to put her front paw over his back. But then seemed unsure about what else to do.
It was so funny to watch! The JRT didn't really see the humour in it, I think...