Symbiosis anxiety is the fear one may not escape from the primitive urge to merge again with mother. If, as is suggested, there is a stage at the beginning of life in both males and females when one feels as if a part of mother, this will establish a feminine quality in one's identity.
While helpful for the girl who is to become feminine, it can threaten the boy's capacity for masculinity. Much of what a society calls masculinity may be, then, an attempt to keep separate from mother's attraction.
Stoller RJ. Symbiosis Anxiety and the Development of Masculinity. Arch Gen Psychiatry. 1974;30(2):164-172. doi:10.1001/archpsyc.1974.01760080024003