why in porn do men often be seen forcing a women to suck their cock ,when this would be awful ,?

Because a lot of men probably find that exciting to watch. Porn isn't intended to be educational, realistic or moral, it's only intended to make money and exciting watchers is the most effective way to do that.

I saw a post on here about how much a woman loves it when her boyfriend pushes her head down there and starts to unzip his pants.

95% of porn is bad - poorly made, unpleasant and unwholesome.

They show men what they want to see. Doesn't have to be accurate, doesn't have to be positive. No laws saying it does.