Well, on the moorland farm we kept the tups in the next field to the ewes for two-three weeks before tupping, so that the girls had their fancies tickled and would all be cycling when the tups were let in. We never had any "failures of the barrier method" (love that!
) - and I'm talking 450+ ewes over a period of several years, with only a single-thickness of stock fence between them - plus 20 feet of galvanised gate that you'd have thought might offer 'opportinities' to diligent suitors!
So I think that one must be breed dependent. Maybe those Hebs, Soays and so ons are just a little more agile and determined than our Swaledales, Mules, Texels and Blue-faced Leicesters!