Why do guys tell dirty jokes around girls they dont know?

Answer 1

It may mean nothing in particular, or it may mean something, but the meaning would depend on the situation and the people involved. Here are some possibilities, which you can think about and evaluate to see if any match your own intuition:

(1) The guys are trying to tease or embarrass the girl;
(2) The guys are attracted to the girl, and unwittingly begin telling the jokes not realizing what a tipoff this is;
(3) The guys have limited intelligence and/or poor manners;
(4) The guys are actually trying to deal the girl in to their friendship circle (i.e. treat her as one of the guys);
(5) The guys have no other way of dealing with girls other than as sex toys (see #3);
(6) They guys believe that telling such jokes makes them seem more masculine and attractive to the girl (again, see #3).