Christians celebrate rape, torture, slavery and genocide.

[Darkstar]

Forced marriage does not necessarily equate to rape

It's immoral.

Mmmhmm

I'm not referring to myself, I'm referring to those to whom this law was given. Was it sinful for them to apply it?

Well obviously not because 'sinful' is defined as "what god says is wrong", not "what is actually wrong". You would think that a being with objectively perfect morals would, you know, fix the moral problems of its followers. He doesn't advise aginst stuff like this though, and even advocates similarly immoral things that, for the most part, people of the day only followed because their morals were not as developed as the morals of today.