The word you're looking for is transactional. Monogamy along with objectification makes sex transactional in the eyes of men.

It won't occur to them that even though their wife is having sex with someone else, she could still have sex with him, he could also have sex with someone else, it may not be an ongoing thing, or that benedits need not always be gains (meaning the benefit could be a reduction or elimination, like pressure to perform or keep his wife happy lest he be dumped).