Along the lines of what Mod-80 said. I heard from the mouth of a grandson of R’ Ahron Kotler the following:
For whatever personal reasons some of the grandchildren of Rav ahron were living by him as children. At one point this grandson had to do a book report and did it on Abraham Lincoln. R’ Ahron walked into the room while he was reading his book and noticing that it wasn’t a sefer asked his grandson what he was reading. The grandson answered that he had a book report to do and he was doing it on Abraham Lincoln. “Who is this?”. A president, he was very honest, they call him Honest Abe etc. Rav Ahron says back “A goy is a goy is a goy”.
The moral of the story is, just because we are mechuyav for darkei shalom to be amicable to goyim does not mean we should start allowing them to sign our kesubos – and there is a reason why we don’t let them do that.