I think the severity will differ from person-to-person and belief-to-belief (morality). Because of her being Christian (assumption on my part), I can understand why she and people would feel the urgency of the situation.
You are correct in the totality of the situation. If he wants to do this, it's his business and he doesn't have to stop. However, because Christians equate porn and homosexuality (not saying he is or isn't gay) with sinfulness, from their point of view one could compare it with another addiction/desire, destroying our bodies (the bible states they are not ours, they are suppose to be the temple of the Lord). If a family member was morbidly obese, it is their business to eat what they want, whenever they want. But because I (representing Christian beliefs), personally, would feel it was detrimental to his health (spiritual walk), I would have the desire to help (but not to condemn) the family member get to a healthy state. Of course, we would also have to assume her dad had similar beliefs towards porn and homosexuality (assuming he is Christian too).
I agree. Why a straight man would look at gay porn? I don't know, but that is besides the point. Ultimately, when we put the morality of it aside and assume it's something her father/mother watch together, it has nothing to do with her at that point.
As to if he is cheating, that is up for debate. I'll say this much: I don't think watching porn is "PHYSICALLY cheating"... but then again, there are more ways of cheating than physical. That would make a great thread topic!!!
I also agree. Condemnation or judgment (of his character) shouldn't be from his daughter but from God (assuming again that they believe in God). Pointing out how you feel something needs to be fixed/changed, be it morbid obesity or how Christians view porn/homosexuality, is not itself condemnation or judgment (to his character) however. I do thing there is a fundamental difference.