I imagine it'd have to get closer to 75% before people actually started worrying about men's graduation rates.
As to the gender gap in the hard sciences 52% of undergraduate Chemistry degrees go to women which arguably makes them under represented, but only relative to their overall dominance. Chemistry is often looked at as a gateway to Medicine so it's arguably a great choice for an undergraduate degree. However, I would argue getting a PHD in Astronomy or Physics is a poor economic choice which is one of the reasons why so few women do so.
PS: Any argument that women can't hack / are unprepared for the hard sciences falls apart based on chemistry numbers.
"I think it will take a bigger imbalance before people start worrying about a gender imbalance in favor of men."
Perhaps. But the majority of the gender imbalances negatively affect women. So let's look at that first.