You see, there's a disturbingly common glitch (as in it's happened to more than one person) where it seems that Connor Kenway, Desmond's great-great-something-or-another, is apparently overtaken by the stresses of being an assassin in Colonial America and just starts jacking it wherever he may be. Like by the river:
Via Anton Procter
"They'll never see me coming."
Or with friends:
"'Become a cop,' they told me. 'It'll be an exciting way to serve your community,' they said."
So, y'know, imagine finding out that your bold, brave ancestor was also a chronic public masturbator and being unable to do anything about it. Also note that this was not the first time in his family line that Desmond encountered such perversions. Yeah, we're not sure how these guys had ancestors in the first place.