What do you say?

Every now and then I will be talking to a male scientist when out of left field there comes this off hand comment. And while not directly offending, it might be slightly politically incorrect. And almost invariably, I will let the moment pass, because either it only sinks in later (making it feel strange to go back to something that might just have slipped out) or because I honestly don't know what the proper retort would be.

Two examples.

Exhibit A
I was talking to a guy my age for the first time. We were exploring future opportunities and without going into details, suffice it to see that I had the upper hand. He needed more from me, than I did from him. And at some point during our conversation he let it slip that I'd probably do just fine in my TT "because you're a woman". My calm response was that while agreed that I would likely be successful, I personally assumed that it was due to quality of my work. I am not sure if he noticed his faux pas at all. It just seemed to slip out and my reply didn't really seem to raise awareness either.

Exhibit B
I was teaching a class and one of the male students commented how this specific topic was really a "women's field". I asked him what he meant by that and he pointed out that I (thanks for noticing) and all of the TAs (all members of my lab) were women. I replied that sometimes it was just hard to find good men (okay this was probably not the best thing to blurt out, but this was my attempt at responding to a weird comment with a joke) and then went on to explain that when you hire people, you just hire the best ones you can get for a job and by chance, all of my recent hires had been women (looking back I wish I had expanded a little more and explained that in any team it is always best to strive for a balance/mix of people with different qualities, sex being one of them - or that based on an n=4-5 you cannot make sweeping statements about a hole field).

So my question is: what do you do in situations like these? I feel like I am always more prepared when I talk to older men. It's like I am almost expecting them to be unaware of women-in-science issues and so it is easy to point out why their assumptions are wrong, or whether they are missing something. In fact, that is also how it often is (at least in my experience) with older men: it's more of a lack of awareness than an outright sexist comment. But I am always caught off guard when it is people my own age (or as in the students' case, half my age!) that make these comments, which touch upon male/female scientist issues quite directly. It depresses me that apparently the bias against women is not going to magically disappear when the older generation retires: it is just as present in the ones to come.
I feel like I should take each of these opportunities to raise awareness, yet I also don't want to become some feminist warrior with a label attached. But how can you change things if you don't openly address them? You don't want to make these things bigger than they are, obviously, but at the same time, they offer room for discussion. Would the male student also have made a similar comment if I and all of the TAs had been male? Would he have said "wow, this is really a male topic"? Would it be okay to let someone know that telling me I'll have a job just because I'm a woman that this is a rude and insulting thing to say?

So help me out here. What do YOU do in moments like these? And what would be the best? Do I let it slide so nothing ever changes? Or is "in the moment" not necessarily always the best moment? Do I collect these stories and instances so that at some point I have a small "awareness lecture" that I can pop in at the start of one of my classes?