That said, I'm talking only about the mechanics of intercourse itself.  Having a big one doesn't mean you can just plow your way in and expect to satisfy her.  Of course women need to be kissed and held and caressed and touched.  And to be honest, it's better for guys that way as well (it raises our level of passion, and helps us get to a good, sustainable plateau, which in turn helps us last longer - which is the ultimate irony in male and female sexuality, that lots of foreplay helps women get there faster, and men get there slower).

But, for most couples the goal (the holy grail, really) is simultaneous climax during intercourse, and the sad fact is that most men are not going to be able to accomplish this without getting a hand down there to help things along, or getting very creative with positions.

Of couse, there's also the fact that every woman is different.  Some women really love penetration, and don't need a lot of other help.  Some women are entirely clitoral, and penetration alone is never going to put them over the top.

But size most certainly gives a dude an advantage.