We had this discussion in the office today and the answer to A was pretty unanimous but to B and C was varied so I thought I'd take it to Mumsnet.
X is single and dating. Meets Y and they are together 6 months.
X then finds out Y is actually married.
A - if you were Ys husband / wife would you want to know?
B - do you contact Ys husband / wife and tell them?
C - would you do it anonymously?