let's go with "pedophilia".
read the posts i quoted - they're very much projecting.
also, your platittude can be applied to many moral situations of varying degrees:
if you cheat on your taxes
if you frequently speed
if you dont donate to the poor/homeless etc etc you have "poor morals which can translate to other parts of your life. it's not hard to get".
if that strikes you as logically sound enough to run with and screw up your cousin's family, go nuts.
I think if someone cheats, it means they are unhappy with the way their life or current relationship is, or too scared to end it for some reason. Some people just think they can get away with it and do, because the significant other will keep on running back anyway. It's still a horribly unhealthy situation for both people, unless there is an agreement for an open relationship (but given the cousin in this case sounds like an extreme Christian - I doubt they have that sort of agreement)
Those options you bring up are highly impersonal bar maybe the speeding one, which CAN have a horrible impact on someone directly, if you speed into another car or into a person. But I think that's very dependent on how someone reacts, you are a jerk for speeding, but if you regret the actions caused pain or death and actually you stopped to help the person you hit, then at least you tried to fix the mistake, if you speeded and then kept on going anyway. You are are a coward or a massive dick, no matter what your excuse was. But people rarely take into account what their taxes do for others, or see much of what their charity will do for people, or even come into contact with the people they effect in those decisions.
They could be a saint, donate to charity, do all the good in the world. But cheating is a hard one for people to get over. Because you have someone who implicity loves you so much, they pledged to spend their life with you, and try to make it work, but the other person...rather than saying "no this isn't working for me anymore" or being honest about their feelings, decides the better solution is to screw on the sly. It's an easy solution which they may think means nothing changes, but it's generally proven when people do cheat, they get more irritable with and can't satisfy their actual partner and so on (so now the other persons needs are not being addressed, because those needs are being given elsewhere) , That doesn't make the cheater an entirely terrible person, but their decision can cause the family unit to suffer regardless. In which case maybe that person is a better parent on their own, or having found a relationship that suits them better elsewhere
People do actually suffer from parents sticking together, when it's rather clear at least one of them isn't interested in the family unit or clearly want to be elsewhere. A friend of mine dealt with a very unstable relationship between his mother and stepfather, and he suffered from depression thanks to pressure placed upon him as result (constantly babysitting his baby brother, arguments, constant drinking ect.)
Sometimes it's actually worse for parents to stick together rather than break apart.