Is sex somewhat painful? For a boy, not a girl? I'd think having somebody sit/squat on your penis would be sort of uncomfortable. That's a lot of weight to put on that area of your body. Also isn't the vagina a tight fit? Wouldn't it be painful to squeeze the penis into it?

No, sex doesn't hurt guys. And it really doesn't have to hurt girls either if the guy is gentle, uses lubrication, and takes his time.

OOOhhh no... In fact it feels wonderful.... and the tighter the better. as long as theres plenty of lube.... I like to squeeze into the Vagina and anus as often as possible.... So not to worry...

Not to disagree but, for a guy, sex CAN hurt. Your foreskin, if you have one, can get irritated especially if you fit very tight or have been working it too hard. Riding back and forth ,dry, on this irritation can hurt. It is also possible to get varicose veins in your foreskin over time making it super sensitive. Also,if you take one of the big three sex enhancers, it makes a much tighter fit so it CAN hurt more. If you have a tight foreskin,(that is...it hurts to move it over the head), it would be plenty painful. So, it depends.
Using plenty of lube and a lot of foreplay, and being patient, solves the majority of tightness problems. Above this, like Susan and John say, it all feels good.

Now, I realize that you are trying to imagine how a woman's weight on top might be uncomfortable. A woman distributes the weight of her body in such a way that it is quite comfortable. She moves very slowly at first until she is well lubricated... then things pick up to a faster pace. On top or on the bottom, it is very pleasurable as you will see..