Thanks for the info..I remember reading how nervous first-time stallions are first introduced to a "seasoned" mare..Boy, I wish someone had done that for ME..LOL.....IOW, I gather it's impossible for the stallion to produce semen unless he is in an excited state, and to do that he has to be on two legs..
They do also have mounting contraptions now that stallions learn to think is as good as a mare.