While we are into familiar discussions, let me toss in a less discussed question.
At least in old editions, the familiar was compared to a spouse. The Gift means that many magi go unmarried, and the familiar is as close as they get. The familiar is really supposed to be that life long soul mate.
In 5ed, there are quite a lot of canonly married magi. Marriage is not as unusual as it once was.
Now, I wonder, how about the magi who has both a spouse and a familiar. Does that make them bigamists? Morally and/or emotionally? Does the spouse in any way diminish the bond to the familiar, or vice versa?
Interesting topic, I think it really depends on the familiar and their form. For a humanoid familiar, this would be a legitimate question. On the other hand someone who ends up with a low Int/Cun magical animal, there would no be such a feeling, since people are well accustomed to the close relationship between a dog and its master.
A Merenita Magus with a Holzfrau might gather a little side eye, whereas a Flambeau with a basilisk.
For example, my previous character, Merenita, was often made fun of by the other Covenant members about having a pretty faced faerie for familiar in a way that the Te maga with a dog was not.
Considering that mages will typically have int +2/+3, I don't see an animal with int -3 as being a fulfilling peer that could compete with a spouse, especially if the animal lacks vocal cords and therefore the ability to speak coherently. At best it'd be like a child.
There is obviously the option of finding a CrAn mage that knows how to stat boost and convince/pay them to up the Int of the familiar, which is probably, in the long run, a good Vis-investement. Also it'd make for an in-game story as such Magi are probably in high demand...
Magi with argumentative spouses and/or familiars may have to discuss this argument with the one or the other.
I would recommend to agree with the magus' player on an Ability - like Charm or Leadership - and request a score of 2 in that to handle their jockeying for position without die rolls. I expect the magus to share his Parma regularly with their spouse for one or two decades, if he doesn't have the Gentle Gift.
Sharing in the experiences of wealthy muslims with their harem might also open the eyes some magi.
Is it bigamy? Not in the eyes of the law or the church. It certainly borders on polyamory, but the familiar relationship is not (hopefully) sexual, and likely not romantic, so it would take a hefty jealous personality trait to have an issue with it. If you want a good pop culture analogy it is like raj and Howard in Big Bang- their friendship was close enough that many people wondered if there wasn't some latent or repressed homosexuality involved, but when Howard wound up with Bernadette there was no friction (or not much).
I would tend to say there are as many answers to that questions as there are magi, and that you should expect someone with the gift to be somewhat non-mainstream. A very christian magi probably would either have a spouse, or a familiar that shapechanges into the opposite sex, but not both. But I frankly wouldn't be surprised to see some magi go for bigamy / polygamy / harem-like approach, which may be rendered difficult by the gift, but probably is helped by wealth, status and the vast power difference between a magi and a commoner, much like the Kings and high nobles of the era who would be married but have official or secret mistresses. and that's without discussing the practical applications of magic. And I'm not even getting into discussing the peculiar hermetic approach of having apprentices as legal property living in your sanctum for fifteen years, with half the apprentices being of the opposite sex...
I dont think most priests would consider it bigamy in terms of how it is religiously defined. But I do think that having a soulmate that you do all your research with, spend months locked inside a lab with and share your most private thoughts with is likely to make your spouse jealous and rightfully so. Even if the familiar cannot speak it is likely that the magus will at one point bind the ability to communicate telepathically or to speak into the familiar bond. In particular telepathic communication has great trouble potential ("have you been speaking telepathically to your familiar this whole time while we were having dinner together?!?").
I also think that said soulmate is likely to feel a certain degree of jealousy if you take a spouse and expect to now spend only about half as much time with said soulmate in order to make space for a spouse.
It is an arrangement that is bound to cause trouble. Which makes it really great to have in a game like Ars magica, since it has really great story potential.
I think it worth mentioning, that I absolutely think that in most situations a spouse and familiar would absolutely be right to be jealous of each other. There is simply only so much time in one persons life, and so much sharing to be done with even just one soulmate, let alone two.
The obvious exception is of course if a magus is engaged in some form of political marriage where the spouse doesnt expect to be a close friend and confidant.
Actually, I'm working on one, but it's still very much in draft mode (and mostly because said prospective spouse has Might - I wouldn't bother for a mundane going through original research / ancient magic integration). I think, if I had to pick between a mundane spouse and a familiar, the familiar would very much win in my current magi's mindset.
I believe technically bigamy is being married to more than one person, and you are not legally married to your familiar.
In terms of relationships there are so many conditional clauses on how they are likely to feel that it boggles the mind- is the spouse another maga with her own familiar? Are they expecting a monogamous or relationship or emotional intimacy? From my understanding in this time period most marriages were viewed as a business relationship ranging from economic partnership to the wife being property of her husband. If you are in Provence or Normandy where the idea of courtly love has taken hold it is accepted that it is impossible to love your spouse, though the idea that the "person" you do love is an animal would still shock them...
Using modern ideas of marriage, (loving, caring, emotionally supportive) in the setting where mages put normal people's teeth on edge, is a difficult concept for me to grasp. That a medieval mage could get married for purposes of procreation, politics, economics, or power does make sense, to me. That such a relationship would be even close to what modern people believe a marriage should be? Nope.
For powerful men, having a mistress was almost a requirement, for married men. They usually had several. Many medieval women would have left their husbands, (they were forced to marry someone chosen by their fathers) but that would have been economic suicide.
So, would mages have married? Very likely. Would those marriages have been to someone they "loved?" Probably not. Would their familiar still be the closest emotional attachment they had? Absolutely. Would they get made fun of (at least by other mages and maga) if they actually loved their spouse? No question.
The other issue is how longevity rituals affect marriage, which is distinct from kept women primarily in the recognition of offspring and inheritance- but a mundane wife is unlikely to inherit from a magus with a longevity ritual (though several mundane husbands might leave fortunes to their wives- especially if the marriage was childless... interesting story potential of a maga who took a longevity ritual to avoid having children at a young age and then winds up married repeatedly...).
Considering that most mages would be considered marriageable before finishing their apprenticeship (due to the length of a mage's apprenticeship) this would lead to a very interesting marital dynamic. I could even see the possibility in some tribunals for it being a parens responsibility to arrange a marriage for their apprentice. then again a Jerbiton with an older apprentice might well have married their apprentice in order to acquire them...