It's scholarships that count, not roster size. Men's lacrosse (maximum 12.6 scholarships) has a slightly higher scholarship limit than women's lacrosse (maximum 12 scholarships). But that doesn't mean that adding men's lacrosse and women's lacrosse simultaneously hurts every school from a Title IX standpoint. You also have to take into consideration the percentage of overall student body enrollment by gender, as well as the athletic scholarships by gender already being awarded in the sports the school plays. Remember, Title IX only requires that athletic scholarships must be awarded, on the basis of gender, in an equitable ratio compared to the overall student body ratio. It does not require equal amounts of scholarships to be awarded by gender. So adding both lacrosse sports may hurt some schools from a Title IX standpoint, while helping others.

Since we're talking largely about ACC schools, I believe Georgia Tech could add men's lacrosse alone without running afoul of Title IX rules. As it currently stands, Georgia Tech has only approximately 35% female student body enrollment, and actually awards a slightly higher percentage (assuming full scholarship limits in every sport) than that to female student-athletes.