there needs to be something that boys are afraid of becoming. The foundations are set early. From John Hart and Diane Richardson [The Theory and Practice of Homosexuality. London: Routledge and Kegan Paul, Ltd. 1981]:

Tomboyism in girls is seen as somewhat more acceptable than sissiness in boys. Indeed, tomboyism is often seen as a transitional phase of female sexual development which many girls go through. Similarly, whereas the term sissy carries implications about sexual orientation and may be applied to older boys and men, the label tomboy does not, and is normally used in reference to young girls only.
(pp. 24-25)

... and the following comes from Ruth E. Hartley [Ibid.]:

The desired behavior is rarely defined positively as something the child should do, but, rather, undesirable behavior is indicated negatively as something he should not do or be - anything, that is, that the parent or other people regard as "sissy".
(p. 9)

At sexual maturity, the connection between sissyhood and homosexuality and the negative way of defining desired male behavior as not-a-sissy come together in a homophobic-sissyphobic- misogynistic pattern: masculinity as we know it.

Related Material: Talking Straight: Reclaiming Hopes for Women's Sexual Liberation
--Lynne Segal