I personally think it's wrong to shame some one for their sexual past but it's ok to judge some one for it. Because deriding some one for their past choices is mean to me, but judging is a more involuntary action.

No matter how you feel about it you have already judged him or her for their past. Even if you don't care about it you are still making a judgement on the situation. The verdict being that you don't care.

Bonus question: Do you think slut shaming happens far too much?

I think it does.