I can think of three places. One is on Mt. Horeb Pike at a church in the middle of a bunch of horse farms. My friend had a wedding there in August a few years ago. It was gorgeous and the weather was atypically pleasant (not too humid for August). I think the church is Mt. Horeb Presbyterian Church. It couldn't be that many things because there can't be that many churches on that road.
The second is the Headley-Whitney Museum. It is a decorative arts museum on Old Frankfort Pike which is also surrounded by horse farms. I have not been out there, but I have friends who have and say it is lovely to have an outdoor wedding.
One of my friends got married in an outdoor garden of a historic home near downtown in Gratz Park. It was the Bodley-Bullock house. Here is there website:
I don't know if it has been done, but the Kentucky Horse Park and Gratz Park in downtown would be nice places too.
The Ashland estate would also be nice and I think they do weddings too. Here is there website: http://www.henryclay.org/grounds.htm
Hope this helps. I live in Lex and any questions you have, I can try to answer.