No point arguing with them. These people are just the puritans of the past -- they've just flipped what constitutes purity. I'm a younger millennial and it's hard enough for me to understand the supposed 'logic' of leftie morality police, and I've been surrounded by them my entire life. If you're a boomer (judging by you having a 38 y/o son), you're unlikely to ever understand the way they think, and it's not worth trying. Just reassure your son that there's nothing to be ashamed of for asking out an adult woman, and encourage him to brush it off rather than trying to justify it to anyone. They can smell weakness, and attempting to defend yourself against accusations is (somewhat validly) perceived as vulnerability. There's nothing you can say that will change their mind, and if you show them that it bothers you, they'll just double down on the attacks.