Juana la Beltraneja got the nickname because she was supposed to be by a friend of her father’s, Juan Beltrán. Her official father’s attempt to divorce his first wife, Blanca, on grounds of her infertility, got Blanca proved still virgin and Henry proved to have difficulties achieving copulation; Blanca claimed that Henry had asked her to lie with other men.
<pure speculation alert> There’s probably no way to know, given how shaky the numbers are even for the present day. But it does seem likely to me that back when women were given little if any choice who they married and divorce was unobtainable that “infidelity” on their part would be much higher. In such a situation she’d be much less likely to feel any sort of obligation to her husband, and less likely to actually find him attractive. For that matter she could easily be angry enough at being forced into marriage as to have affairs & children with other men just for the sake of taking what revenge she can on him. It just seems logical that a woman who actually likes her husband would be much less likely to sleep with another man.
Given that men back then were also pressured into marriage (if not to the same degree), I’d expect more infidelity on their part as well.
<also pure speculation alert> On the other hand, the consequences if you were caught were much higher back then. Even when divorce was available, it often led to total ostracism for the woman. In some societies, adultery could get you killed or various other unpleasant punishments. In others, you’d be considered to be damned to hell, and that was a big deal. So all of those factors might have kept down the numbers of women having affairs.
On the third hand, without reliable contraception, an affair would be much more likely to result in a child. So even if there were fewer affairs, there might have been more illegitimate children born from them.
It’s an interesting question and one that goes through my mind whenever I see attempts to trace one’s ancestral lineage, like with the show Who Do You Think You Are? that does so for ‘celebrities’ or anytime an Ancestry.com commercial comes on. Sure, there are documents and records which can trace marriages and birth records but by the time you go back five generations or so what are the chances that you’ve actually traced the true genetic route.
There are more than a few cases of men paying child support for children who are not biologically theirs nor did they adopt them. In some cases these men weren’t even married to the women, but still on the hook for child support. The legal system seems to in favor of the child getting support money from anyone especially if the biological father can’t be found.
Google “men paying child support for kids that aren’t theirs” for more info.
That you are not the biological father. But that you are still the legal father unless you can get your parental responsibilities rescinded and often the emotional father if you are married to the biological mother and spend years accepting the child as “your own.”
The law in all fifty states establishes the presumption that a child born to a married woman is the child of the woman’s husband, yes. But that’s only the presumption, and it may be challenged in the majority of states, with different rules for standing in different states.