My first instinct was to point out that it wasn't really unacceptable for a woman to rule in her own right outside of France - as I discussed recently in What Happened in the Late Middle Ages that Suddenly Allowed Queens to Be More Palatable for the Ruling Class in the UK to Support as Regent?, for the most part, thrones were able to be passed from father to son without any openings for daughters quite frequently. While I noted that it was unacceptable in France, it took several hundred years of the Capetian dynasty before it even became a question. In other countries, when a woman inherited the throne, she inherited the throne. While they were forced to wait behind all their male siblings to inherit, reigning queens were granted all of the same powers as kings and, as far as we know, didn't face male councillors undermining them or telling them they're inferior or anything else that you might come across in a period drama meant to make you relate to the sexism the heroine has to deal with.

On the other hand ... from a standpoint that's less defensive of queens, you very much have a point. In the Middle Ages, when women inherited thrones they frequently didn't get to just rule. As I wrote in another answer on queens regnant before Isabel of Castile:

Navarre in particular had had several medieval queens: Juana I (1273-1305), who came to the throne as an infant and allowed governors to rule Navarre when married outside of the kingdom; Juana II (1312-1349), who ruled jointly with her husband; Blanca I (1387-1441), who did the same; Leonor (1426-1479), who unfortunately died almost immediately after being recognized as queen; Blanca II (1424-1464), who was imprisoned by her family when others declared her queen and was never able to act on it; and Catalina (1468-1517), who also ruled jointly with her husband, post-Isabel. Léon also had a major one before Isabel: Urraca (1079-1126), who ruled jointly with her husband. Aragon had Petronilla (1136-1173) and Castile had Berenguela (1179-1246), both of whom abdicated in favor of their sons. (And outside of these examples, Iberia had strong traditions of officially mandated queen-lieutenants who ruled in their husbands' stead when the king was ill, on crusade, in battle, etc.) When I say "ruled jointly", what I mean is that it was accepted that their husbands should also be crowned as a ruling monarch - elevated to kingship - rather than seen as consorts in the way that queens who married into a ruling family were, and this can encompass anything from "the king and queen were truly equal co-monarchs" to "they were both crowned, but he acted as head of state and she acted as a consort".

Clearly, while the medieval Iberian kingdoms had little problem allowing women to inherit the crown, it was not really acceptable for a woman to rule in her own right. As I discussed in this Tuesday Trivia post about Berenguela of Castile, women were seen as acceptable conduits for royal power, rather than wielders of it: good royal women could make their husbands kings, or they could transmit the bloodline and right to rule from their fathers to their sons, but they didn't actually try to have power themselves. Back in the pre-Muslim Visigothic kingdoms, it was even found useful (in dynastic shifts) to marry a predecessor's widowed queen as a symbolic tie with a previous, clearly legitimate ruler and conduit of his legitimacy and power.

Regents were historically in a much better position to wield power because they weren't claiming the right to it. There was a clear end date to their reigns, typically the year of their oldest son's majority, which made them less of a threat - they also could make claims to acting as nothing more than a stand-in, effectively putting the kingdom in stasis until such time as they could be relieved. But as female rulers with less inherent, inherited legitimacy than queens, their historical reputations were extremely fragile and vulnerable to misrepresentation, because it was fairly safe to blame them for anything that had gone wrong during their regencies. My go-to example is typically Isabeau of Bavaria (1370-1435): the wife of Charles VI of France, who developed a mental illness that left him incapable of ruling for long stretches of time, she was appointed head of the regency council by him and presided over the French court during (some of) the Hundred Years' War. While she theoretically had the ultimate power as head of the council, though, she was ultimately dependent on the dukes on the council doing what she wanted, and they did not always. They were divided into two factions, and it was her job to mediate between them, which she did successfully. However, long after her death a black legend sprung up around her - it was said that she was a seductress in an affair with one of the dukes, that she gave orgies, that she wore gowns open down to her stomach, etc. - and she would be used as an example of a Bad Queen in French history in the eighteenth century. Even until fairly recently, it was assumed by historians that she was a weak ruler who caused the ignominy of the Treaty of Troyes, which disinherited her son in favor of the children of Henry V.

Even royal women who weren't queens could garner suspicion by being too powerful. King Fernando of Leon divided his kingdom among his children on his death, and one son, Sancho II, decided to gain the rest through conquest. His sister, Urraca (1033-1101), stood against him when her city was beseiged; he was assassinated, and she went on to stand with and advise another brother, Alfonso VI, who became king over Castile and Leon with her assistance. Later tradition put the two in an incestuous relationship (instigated by herself so that she could be queen) and accused her of having Sancho killed (evil not just because it was murder, but also because he was the oldest son and rightful heir).

It's tricky, when you talk about queens, because you have this balancing act. On the one hand, it was definitely not unacceptable for a woman to rule (broadly, across Europe), and people didn't simply rise up and tell women who inherited thrones that they didn't deserve it. Royal women in general had quite a lot of social and political power in their contexts despite societal sexism. On the other hand, there was quite frequently a response to these powerful women, either in their time or afterward, that purported to be about their individual capabilities but was entirely rooted in their gender, which makes it clear that on some level people did think it was unacceptable for a woman to rule, whether in her own right or as a regent.