For your edited question:

Haraldr Sigurðarson harðráði (his father was Sigurður Halvdanarson sýr) was the father of Magnús Haraldsson and Ólafr Haraldsson kyrri. Old Norse names were patronymic (or, occasionally matronymic) rather than having surnames that transferred from generation to generation.

Can you give a better idea of where you mean in Europe?

For example William I of England was a bastard who succeeded to his fathers dukedom of Normandy, but was an only son and despite his father declaring him heir and having fealty sworn to him he faced some rocky early years before establishing control.

and would it be seen as fine for a noblewoman to take male sex slaves?

I'm not sure where male sex slavery came into this but generally no. Under Germanic law for example adultery was seen as an offense against the husband's authority over his wife. The same was not true in reverse; men could not be guilty of adultery. And while extramarital and premarital sex were frowned on by the Church, men still took actual concubines (eta: this was completely legal.)

See Chapter 8, Noble Strategies: Marriage and Sexuality in the Zimmern Chronicle

It has almost always been different for men and women as titles and estates passed through the male line, so paternity needed to be protected.

I'm not sure when that loosened, but it probably had to do with declining infant mortality. The more liberal attitude of "an heir and spare" allowed noblewomen to take lovers after producing a male heir and a male backup.