What time period are you referring to here? and what do you mean by modernized sexuality?
There was never a time when everyone followed some rigid code about sex. It is true that in the middle ages the topic was more hushed up than it is now, but people have never been particularly virtuous.
That is partly true. Obviously, yes, there were philanderers. But it is rare that I see RPs of conventional marriages, stable relationships, or anything like that. Every RP is some horribly complex drawn out romantic love affair. Which is just bogus.
You are right that not everyone followed their moral code, but they were mostly discrete about violating it. Consider even the Arthurian legends by Mallory: Lancelot will NEVER confess to actually banging Guinevere, and that's even in a popular hyper-romantic novelized form. Courtly love at least pretended to be chaste.
However, you are also partly wrong. There is no evidence to suggest that "philandering," or any sexual activity, is consistent across time and culture. Quite the contrary, we have good reason to believe that cultural mores play a very, very large role in regulating behavior. Does that necessarily mean the middle ages were more chaste? No; quite possibly the opposite. But to simply suggest that our sexual behaviors today would translate to previous times in terms of their functionality or frequency is untenable.
