If your husband has experience riding motorcycles, then you'll love riding across Tuscany!
I found a San Pancrazio in the province of Arezzo, is that the one you're staying at?
While it does to be in the hills, it is most definitely not in the mountains so I think the roads will be quite normal roads across all of Tuscany, which I consider safe to do in motorcycles. Not too steep between there and the main towns in Tuscany -- the steep roads are once you're in the mountains and going to remote towns uphill. It just means you need to take it slow and have the power to take them, which I think most Harleys can tackle. Most of the roads even through the mountains pass along the valleys in between so you're not necessarily climbing uphill.
The women who runs your villa might have had a bad experience with a motorcyclist to be afraid of or for them... to be honest, there are tons of motorcyclists here, many who love to doing the mountain passes on the weekends. Unfortunately, they tend to be a bit too daring --- often taking the turns too fast and too wide and invading the opposite lane. These are the maniacs I worry about myself when driving along those mountain roads because you can seriously find them in YOUR lane as you make the turns and not see them coming at you. There have been many accidents on those roads, as you'll witness with flowers or ribbons on polls along these roads. So - of course you need to be cautious as you drive, but most roads can be done by motorcycles.
If your husband has experience and does not drive in this reckless manner, I think you will particularly enjoy doing the roads in Tuscany and exploring the region this way!