What you are talking about all comes down to your own culture, life-style, and view of things, for some people love and sex are so separate that they know it doesn't matter how many their SO has sex with, because they know their SO only has love for their SO and no one else.
On the other hand, you have those that view sex and love basically as strongly interconnected, so much so that you should only ever have sex with the person you love, no one else, and if you have sex with someone else that means you no longer love your SO.
In the case of Zenith and - I forget his name... - her husband, they are basically on complete opposite ends of the spectrum from each other, Zenith is very religious in her views of sex, love and marriage, where she believes you should only ever have sex with the one you love and plan to marry - I forget if she had him wait until they were married before she let him have sex with her or not.
While he didn't really care about all of that, to him sex was just sex, while love was something entirely separate and something I think only Zenith was able to make him feel. To him, sex was just a means to relieve his frustrations and have fun, nothing more. It wasn't until after he married Zenith that he started doing his best to control himself and remain faithful to Zenith despite having sex with only one person was entirely against who he was as a person, and then he inevitably ended up failing to stay faithful once one of his previous sexual companions (maid) - reluctant on her part, as I believe I remember she was against it at first, but then just reluctantly accepted it - from their adventuring days came to live with them as their maid.
He obviously betrayed Zenith, and what he did was obviously wrong, we're likely not disagreeing on that part, where we seem to disagree is when it comes to whether him having had sex with someone else is a sign of little love for Zenith or not, I'm of the mind that it changes nothing in terms of how much he loved Zenith, that he loved Zenith immensely before he had sex with the maid, and loved Zenith just as much as before, and even more so because she decided to forgive him and give him a chance to prove himself a changed man.