It's not the term itself that has caused the outrage, but the implications of what he said. There would be nothing controversial about me saying, "some women falsely reporting rape makes it more difficult for legitimate rape victims to get justice."
The outrage comes from the suggestion that if a woman is raped, it is impossible for her to get pregnant. The implication of that statement is that any woman who gets pregnant and claims she was raped is either lying, or (subconsciously) actually wanted to have sex at the time. I don't think it was anything to do with husbands and wives. It was also ridiculed on the basis that it was scientifically inaccurate, despite this politician being on the US House Committee on Science.
Unfortunately, there are still people in the world who believe that anything other than a woman being dragged into a dark alley by a stranger isn't "proper" rape. Some people, for example, are of the opinion that if a woman invites a man back to her house, that is evidence that she consents to sex, and therefore rape in that situation either isn't possible or isn't as serious. Or that a woman that dressed or acted in a particular way was asking for sex and couldn't have been raped. It's in this area where people can talk about different degrees of rape, and phrases like "legitimate rape" can emerge.