I would cheerfully watch Bakemonogatari (and its sequels) with my partner - and indeed do. I actually find it less offensive to my inner feminist than most of the output of Hollywood these days; the girls might be constantly sexualised but at least they have some level of agency and personality beyond being the backstabbing femme fatales and personality-deficient love interests which still represent most women on television and in the cinema in the west. In 2015. In films promoted as showing 'strong female leads' (a claim which Bakemonogatari most certainly doesn't make to begin with).
I gather that the original question was about the Mayoi scenes, which I've always interpreted as more comical than sexy. I've never found them titillating in the slightest, whereas Tsubasa's opening sequence and the overtly sexual moments in Nisemonogatari and later episodes are obviously intended that way. And most importantly, I don't think Mayoi herself feels upset by the teasing - or at least I don't think she finds it any more upsetting than when he makes fun of her in other ways.
I'm not saying that to mean it's ok to shame and objectify someone if they're too immature to understand, but at the same time it's nonsensical to assume that someone so young is going to see things the same way an adult would. It's not as though the things being depicted are black and white cases of sexual abuse. I have a younger relative and he's at the age where he thinks it's the height of comedy to pull his pants down or have his pants pulled down. He has no idea what sex is, and doesn't care. He's just being a kid who has picked up on the fact that adults consider showing their bottoms rude without having the slightest idea why that might be.
(I have no idea how people can raise children in our conservative society without making peace with the fact that adults and children see sexual material in completely different lights. I'm mentioning this because it's getting to the point where any adult interacting with a person under the age of twenty is entering a minefield where anything they do is under scrutiny for potential perversion.)
I was once Mayoi's age myself, like most of us, and back then I'd have found the same thing embarrassing without understanding why I was embarrassed. It's the opposite of why my kid relative finds it so funny, and it's because women are (sexistly) socialised into feeling shame about their bodies long before they develop any sense of what they actually do and don't like for themselves. Girls who don't act shocked when someone sees their pants or breasts are regarded as bad girls, even though it's not their fault and British television is full of girls showing their pants or breasts at all times of day. Neither pants nor breasts are sex organs. I interpret Mayoi's reactions as being driven by the same emotions I felt back before I learnt about my own sexuality and understood what a jumble of nonsense our social norms actually are. She might have come to decide that she was uncomfortable showing that kind of thing later on, but even then it's barely worth remarking on next to the severe sexual abuse and misogyny women experience and see in the media every day.
Moving on, Arararagi is old enough to make his part in the teasing questionable but I've always felt that he sees Mayoi more as a younger sister and (most importantly) he'd be devastated if she was genuinely upset by it. I live in the real world; I've been groped and treated in misogynistic ways and actually, in most cases girls brush that stuff off because it's (tragically) an everyday event. Bakemonogatari makes it clear that Arararagi's activities are morally wrong, and the girls are shown to have the agency to tell him so themselves when he crosses a line without bottling it up inside and turning it into a frustrating reminder of how unjust the real world is. NisiOisiN's world has a more relaxed depiction of sexuality than normal; much less irritating than seeing girls screaming with shame and blushing furiously the way they do in most other fan service titles.
I have a major problem with the inconsistency of UK law in how it views the (intentionally) bizarre sexualisation in the Monogatari series and other fan service titles as acceptable when a harmless non-sexual girl/girl breast grab is too obscene to be rated in Code Geass, but that's because our country is silly. This topic (and Vash's recent bump) show that people have no idea where the line should even be drawn when the rules are being applied in such a haphazard way.
Sorry, I rambled a bit. I'd find it more offensive in a lesser series, but it doesn't bother me at all in Monogatari where I'm reasonably convinced the original writer knew what he was doing.
R
