Good question.
Lets think it out.
Estimates of Lesbian population (excluding bi's) are approx 1% worldwide(pure lesbians) but these are self proclaimed. We can assume that in many places people will hide their inclinations. But even then pure lesbians who will never love a man will be limited to max 5 times (extreme, not conservative estimate) the self proclaimed population.
Also a girl liking a boy is something that can be ignored till 4-5 years. Although there would be tendancies even then of that, but for our purposes, true liking does not really happen in infants. We can thus discount all infant girls.
The infant population of girls can be put at around 3.5% worldwide(assuming around 4.5% for males) and in a population growing younger. (based on data on 7 and below at 10% overall)
We can also add 0.5% extra to include
1) Medical cases who are not capable of thinking, loving, or movement. We can also eliminate some select minority population who loves no one for no particular reason.
In all i would put the probability of a girl liking a boy (Note the vice versa probability is NOT the same) at around 0.91