It's funny that. When women are underrepresented, it's an awful injustice (which it is). Yet when men are underrepresented they're lucky because of all of the women around them...
It's not just that. Societally imposed gender roles allow men more freedom to choose to study what they want, but we still force women into certain fields (education, social work, nursing, etc) and exclude them or mistreat them in other fields (engineering, medicine, the business sector, etc). Men aren't underrepresented in fields like nursing because of a lack of choice - in fact it's the very opposite; they consciously move away from them and aren't prevented from being successful in doing so by archaic, patriarchal attitudes.
Underrepresentation in the context it's applied normally probably isn't even an appropriate word here. It's a completely different reason as to why women are underrepresented in the commerces/sciences (not so much in undergraduate study, but in the actual work force), though fortunately the gender/work paradigm is changing (but slowly). That's why it's not comparable injustice.