Just empathize. If a woman says something offends them, believe it rather than arguing with them about how they're wrong to be offended.
Maybe this joke doesn't offend you, but consider your experience. You haven't had to live your life as a woman putting up with a cavalcade of mostly-harmless sexist jokes, slights, and condescension. For a woman in that situation, the joke could really smart, especially since it's being made in light of a woman breaking into a male-dominated field. If your life was trying to break into a female-dominated field, you might not find it as funny. Or you might, because society isn't systemically set up to deny you access to giant swaths of the professional world. CONSIDER THIS DIFFERENCE.
Ultimately, you thought the joke was funny and you consider yourself a feminist. Great! You're allowed! But when women show up and tell you the joke offends them, don't say "nuh uh" - consider their experience might be different from yours. Consider that pervasive sexism might have made them sick of this shit. It's obvious it makes you uncomfortable to deal with the dichotomy of liking something a little sexist while simultaneously considering yourself a good person. Well, hash that shit out on your own time, son, don't spar with women who took umbrage to the joke. YOU'RE TAKING UMBRAGE TO PEOPLE TAKING UMBRAGE. YOU ARE THE THING YOU HATE.
In general, a man explaining to a woman how they shouldn't be offended by sexist content is not very feminist, sorry. It's actually worse than a sexist asshole telling a woman the same thing because at least then, they can consider the source and dismiss the person. But you're going around saying, hey, I'm on your side, but also "if you're outraged over this - you need to get out more." AND THEN acting pouty when people call you on it, you're doing feminism a disservice, yeah, I will go so far as to make this sweeping statement.
See: all these tactics that you're using to silence criticism: http://geekfeminism.wikia.com/wiki/Concern_troll http://geekfeminism.wikia.com/wiki/Even_women_find_it_funny http://geekfeminism.wikia.com/wiki/You%27re_being_emotional http://geekfeminism.wikia.com/wiki/Pick_your_battles http://geekfeminism.wikia.com/wiki/I%27m_a_feminist_but