It is all right for the stallion to mount without an erection, particularly when just learning the mount. Some prefer to “test” the stability and feel. It is paramount to have a mount that is secure and does not move or make excessive noise.
I heard that performance anxiety from an unstable mount is extremely common.
My colleague Seamus comes from “beyond the pale” in Ireland, meaning somewhere in the middle. Ireland is a top breeding country, and Seamus once told me about going down to watch the (is this what it was called?) studding live and in person. Here’s a couple of Irish studs. He reported that when he watched, there wasn’t any artificial insemination going on, it was actual horse-on-horse action. He did say that they bring in a female donkey as a kind of fluffer, before the mare is introduced. Apparently the donkeys really make the studs’ blood hot, so that they’re all set for the main event.