Can someone help me understand????

I do NOT understand the mentality of a cheater/liar...... I believe if your going to tell someone your monogamous with them and act this way then do it....!! Why would you want to hold someone eleses possibility of "true" happiness w/ someone who is honest... Why not just date multiple women and be honest??? I don't understand this way of thinking???? And why people lie... Huh .. opinions or enlightenment on this matter..... Thnx