It's arousing! 

I'm especially drawn to a man's hands and arms anyway, and to watch
a man do that is enticing. It's nice to watch the expressions on
his face, knowing he's becoming more and more aroused while I sit
there and just watch...smiling.

We women weren't made like men, we're not as receptive to optical
stimulation as men are. Not too many of us can begin to become aroused
just by watching a man walking down the street. It takes a little
more than that for visual stimulation.....i.e., watching him stroke
himself.

I've only masturbated in front of my wife a few times. We like to
do it in the shower once in a while but a few times my wife couldn't
find a comfortable position so she was happy to just watch me stroke
my soaped up cock for a while then squirt on her. Also sometimes she
will kneel down and suck me in the shower but won't let me cum in
her mouth so when I'm ready I will pull out and take over stroking
as she watches and waits for the cum to squirt out...