Why are some men unfaithful?

Readers, I need your help. WHY DO MEN
LIE!!!!
I have been in a relationship  for six years
with someone who lives in Spain. He has not
always lived there, he moved there three
years ago, and since he moved i have been
trying to get a visa to meet him but all
attempts have failed. A year after he moved
to Spain he had a child with a Spanish lady. I
am only aware of this because my sister also
lives in Spain and she found out. When i
confronted him he, he said the child was the
result of a relationship  he pursued in order to
obtain permanent stay within Spain, in order
for him to bring me over.
On the 1st of January this year i asked him to
tell me the truth and stop messing me
around; should i continue to wait for him, or
should i simply forget him and move on with
my life. He begged me to wait....and i did.
However i told him that he needed to call my
father and he did, which reassured me for a
while. We do not talk on Facebook, and i do
not use the social networking site much
myself, however, today is my friends birthday
and i couldn't find her number after
swapping phones a while back so i went onto
Facebook to send her a birthday message.
When i logged in what i was greeted with was
more than a surprise. I saw my boyfriend
with the mother of his child sharing a
passionate kiss. My boyfriend wished her a
happy birthday and called her THE LOVE OF
HIS LIFE. To say i'm heartbroken is an
understatement. What i want to know is why
are men so unfaithful, why hasn't he kept his
promise, and why did he string me along
despite me giving him the option to leave
our relationship  amicably.