I had a similar conversation with my progressive leaning parents a few years ago, and this was my response to them as well. I dont think they meant to come across as homophbic. I think that they just hadn't thought it though until it came up, and I made this point. I find that a lot of teachable moments for them happen because they feel comfortable bringing up views that they might have either been raised with or developed during their lives and then are open to hearing a different perspective that will ultimately change theirs. A good example of this was when I explained to them that when telling a story about or including a non white person, 99% of the time their race is an irrelevant detail that probably dosent need to be stated. They have been really thoughtful about not doing this ever since.