Offensive?!? First of all, I thought it was a very cute sign and very personal which made it even cuter. Second, your friend would have been more offended sitting on a soaking wet toilet seat!
Although, this brings me to something that has been haunting my thoughts; my whole family puts the lid down, and sometimes guests come over and they don't put the lid down-out of habit from their own homes-or just because. Here's my haunting thought; do people think it's unhygienic to touch someone else's toilet seat lid...and that's why they don't close the lid??? My toilet is clean but I wondered if it was one of those unspoken things. Ever since I learned about how "toilet matter" is spread when one flushes, I have been a stickler about keeping the lid closed.