NTA the extra effort your boys had to take just to see her is what makes this spying and wrong. Like you said, she had a reasonable expectation of privacy and seclusion. And yes, boys will be boys and curious about the female body, but that cannot be an excuse to justify this type of violation. Your husband is the asshole for that, that kind of thinking can lead to bad places. Talk to your boys, explain to them why this was wrong, dont just tell and punish. Punishment must always come with a why if its gonna be a true learning experience. And If your husband truly believe in the "boys will be boys/curiosity bit, then when they're a bit older, after he's had the full birds and bees conversation and talked about respect, he can get them a playboy.