at any school that doesn't have women's lacrosse, it's not valid. they could add them side by side and be fine. even the ocr basically said as much, as have numerous court rulings that have addressed title ix.
for those that do have women's lax, their refusal to add men's at the same time or shortly thereafter confirms their lack of interest in doing so. the women's add would meet a test that courts have consistently followed when ruling.
and then there are those universities who are substantially near/at/or past proportionality, who also do not add men's lacrosse (or other men's sports).
you can't prove a negative, but actions are damning.
the proportionality test is no more a safe harbor than the second prong (or the third supposedly, though that one i wouldn't want to have to prove without a stamp of approval from the ocr). and actually has never been defined, that i'm aware of. plus, most schools aren't proportional anyway, yet they go about their merry way every year.
one reason brown univ. lost was because their proportion, while on the correct side of 50%, wasn't close enough. the other was because they were taking opportunities away from actual athletes, and thus were basically failing the 2nd two prongs by that action.
ADs don't want to be distracted from the money machine. that is the only "valid" reason they have.