Two albino individuals marry and start a family. their first child has normal pigmentation. does this indicate infidelity and why/why not?

It would indicate infidelity, yes. Simplistically speaking, albinoism is caused by both of the genes dictating pigment bring recessive - if just one of the alleles allowed for pigment, that is the one that would have dictated the phenotype (physical appearance) of the person, because it is dominant. Thus, we can conclude that neither parent has a pigmented gene to be able to pass on to a child.