Masturbation may increase fertility during
intercourse. A 2009 Australian study found
daily ejaculation to be an important factor
in sperm health and motility. [34]
Female masturbation alters conditions in the
vagina, cervix and uterus, in ways that can
alter the chances of conception from
intercourse, depending on the timing of the
masturbation. A woman's orgasm between
one minute before and 45 minutes after
insemination favors the chances of that
sperm reaching her egg. If, for example,
she has had intercourse with more than one
male, such an orgasm can increase the
likelihood of a pregnancy by one of them.
[35][36] Female masturbation can also
provide protection against cervical
infections by increasing the acidity of the
cervical mucus and by moving debris out of
the cervix. [36]
In males, masturbation flushes out old
sperm with low motility from the male's
genital tract. The next ejaculate then
contains more fresh sperm, which have
higher chances of achieving conception
during intercourse. If more than one male
has intercourse with a female, the sperm
with the highest motility will compete more
effectively.
ref : wikipedia