I apologise in advance if I have violated any woman's modesty
with usage of my words.
what a load of bull$hit! that is one of if not "the" most
ridiculous things anyone has ever said. The penis head is bigger
than the shaft thus forming a ridge. The vagina is a hollow
cylindrical carpal tunnel which means inserting the penis
head(dome) in first already means that your pushing your
competitors semen upwards into the vulva. Once at the deepest point
of the vagina during intercourse the inwards motion forces the
semen deeper into the vagina and likewise during outwards motion
the ridge(due to the head being bigger than the shaft) may scrape
back whatever semen that coated the entire vagina during the
previous ejaculation process(yes it can get messey) but that
becomes insignificant once again, because you already forced the
previous semen more into the vagina track because of the thrust of
your dome. ultimately the ridge does not scrape off the competitors
semen. Let's consider this scenario, a male with a smaller penis
has an intercourse ejaculates into the female's vagina then a male
with a larger phallus has intercourse with that same female who
just had been fertilated. The male with the bigger dome if anything
at all has only aided in making the previous competitors semen
fertilate the female. This is just my rebute to your "theory" on
the purpose of the ridge.
these are my opinions hereupon and is subjective therefore reader's
discretion is advised
that is so not true, during ejaculation the sperm is shot
through the urethera into the female's ovary. bluntly speaking the
size of the machine does'nt matter but the range and the ammunition
matters when it comes to a male's virility.
On the same vein, a longer phallus(which is highly determined by
the shaft) means being able to penetrate deeper into the vagina
especially the cul-de-sac. 
And... most importantly the girth! the prostate size is more
influenced by the girth. The thicker the penis the more stretching
it does on the clitoris the more aroused the woman becomes.
once a woman becomes accustomed to a thick penis nothing else
becomes close to satisfying her orgasm. Hit the G-spot man!
believe me I've realised this first hand. 
all of this I've learned from experience, when it comes to penis
sir... the practicality is more enlightening than theory. 
size matters? yes it does, but that is highly subjective.
A female with bigger busts does'nt mean that she can supply an
infant with more mothermilk. And a female with big hips does'nt
mean that she is more likely to conceive and deliver an offspring.
bottomline the nubility of a woman is'nt based solely on her
assets. (I'm pretty sure the womenfolk of sgforums are going to
applaud me on this, but hey this is nothing but a fact)
