I agree, asking the F/A and then checking reseat options is good. If you notice it in enough time (like before take off) you should alert the F/A and the agent. They may have to remove the passenger.

I was once working a flight (SFO-JFK) in which a passenger went in his pants. He was a very distinguished looking gentleman and was silently horrified. We actually turned the plane around and went back to the gate to allow him to shower etc and fly later. Anything could be rearranged (meetings etc.) to avoid embarrassment.