Thank you for the kind response. While I don't feel entirely compelled by the framework that would characterize it as internalized sexism, I think the way you put it really helps in viewing the situation differently and emphatically, ie, making an effort in understanding that I'd be stopping a female training partner from improving as much as any other person in the gym by letting my own discomfort get the best of me. You're right, they're there for that specific reason, and everyone can voice their disatisfaction with someone who's going harder than they should. As simple as it is, it can be ignored due to the violent nature of it all and not wanting to deal with those irrational feelings of having done something culturally taboo.
I do think that the process of adapting to this understanding may be a slow one for men with this cultural conditioning, but that's when talking about it helps so much. The feeling that you did something that goes against your upbringing (punching or kicking the opposite sex in the face) leaves a foul state in your mouth regardless of any rationalization, but I'm sure that it can be dealt with progressively by knowing how the consequences of this attitude are so frustrating for female martial artists and how unbalanced it ends up being for everyone involved. Both parties need to understand the other's position and needs, and work together to even things out. That is to say, just asking to hit harder might not be enough to really ingrain the idea and erase the discomfort in some men, but it should at least be the start of a conversation that solves the problem.