Yes, and here is why:
Modern Logic, as it currently stands, does not operate on itself. The speaker must identify, in the end, if a premise is true or not. Therefore, for most of us, we believe that we are in a world wherein your grandparents had to have had sex (or otherwise impregnated) to have your parents, so that you parents could have sex (or otherwise impregnate) to have you as a child so you may exist.
Logic does not interfere here, all is fine. You are trying to use logic as a rule for determining the truth of a statement without deciding on a premise or what the truth of that premise is. I could do the same thing you're doing in that post but with statements such as "grass grows, does it not?" or "the sky is often blue, is it not?".
Also I find it funny that most of the posters in this thread are just going "lol my grandpapa had sex and idk how you think not but ur dumb for disagree! >

)". The OP isn't an idiot, he just is applying logic incorrectly I think.