It just becomes too much... and you want to stop for a moment. Shortly thereafter.. I usually go back for more.
During oral... I do worry about hurting the guy... eh, irrational fear of crushing the guy in a vice-like grip with my legs.
In regards to sex... if I can push the guy away, I try. However, sometimes that isn't possible as he's in the midst of his own pleasures... and so it becomes so much to where all I can do is cry out or burst into laughter.
Just one of those things...