If by “having four boyfriends” he means she was screwing around around, that would considered scandalous in a lady of her generation.
But back to Thatcher: If she had four serious boyfriends, but kept it very discrete, than that seems to adhere to the norms of her time. I think people did have sex in the 1940s. But if they kept it under the radar, it wouldn't be scandalous, and for people (us) to find out about it more than 50 years later probably shouldn't be scandalous either.
Barone seems to indicate that this falls into the "lies and omissions" category and that strikes me as needlessly judgmental.