What does the '欢' in '苟且偷欢' mean?
「偷」has a less negative meaning (than stealing) of finding time (to do something), which is the meaning in this context. For example,「偷暇」means to find a (fleeting) moment of leisure.

「苟且」, in this context, is more accurately translated as perfunctory (doing something to attain minimal standards, or doing something with minimal effort), so「苟且偷歡」is to be content with temporary happiness.



EDIT: Since the OP has posted the context of the phrase as


  王八妻子与单身雄蛇便苟且偷欢
  
  (My) bastard wife and that sneaky bachelor "苟且偷欢" whenever they can


then Tang Ho's point about illicit relations is the correct interpretation:


  (My) bastard wife and that sneaky bachelor jump into bed whenever they can