Watching porn without overindulging in it, obsessing about it or it taking all your time shouldn't be a problem. Porn does set up weird and unachievable expectations about human bodies, so keeping that in mind would be good.
You get the negative effects you mentioned just by watching. It will slowly deform you mentally and create an actual physical adiction. You can research about the adiction part, the neural pathways get rewired. And although you are watching soft stuff, you should realizar that it will gradually stop givig you your fix and push you to heavier and even more harmful stuff.