Yes, that's the only reason, sure...but only if you happen to ignore that two of Buffy's three long term relationships were with creatures who by their very nature made said relationships very much a matter of public safety. If in real life someone insists on dating a serial killer, do you think her friends would be likely to say "Okay" and not really care how this relationships goes? And I also have a major problem with disagreements being described as "judgements". So Xander thinks Buffy might not be right in every situation? Big deal. Buffy doesn't think Xander is always right either, is she a terrible friend too? She has sometimes criticized the romantic choices of her friends but oddly enough no one describes this as shaming or judgement. She thinks Giles having a girlfriend is "gross" because he is "so old". Ageist judgemental bitch! See where I am going with this hyperbole?
But it's not real life, and as it's been pointed out before, Xander doesn't have any problem dating Anya. She's not a current demon, but she also doesn't show remorse for her past actions or much regard for human life for a very long time. If we are talking about real life, doesn't she qualify as a psychopath, similar to a serial killer?
And as for Buffy criticizing Giles, I don't think that's very relevant. It's played for laughs exactly because it's clearly irrational of Buffy to say that about an adult man. Xander's criticisms are actually taken seriously, both in the text and in fandom.
Also, a lot of the time, it's not even that he doesn't approve of her relationships, it's the
nature of his disapproval. He constantly slut-shames Buffy, and that's red flag number one for me. If his disagreement was only because he's looking out for their safety, that's another thing. But in multiple instances, it's clear that he's jealous and his behavior is for personal reasons. Like in 'Surprise', he even talks about a fantasy he has of "winning" Buffy like a prize away from Angel:
GILES: They should be back by now.
WILLOW: Maybe Buffy needed a few minutes to pull herself together. (sad) Poor Buffy, on her birthday and everything.
XANDER: Hmm. It's sad, granted. But let's look at the upside for a moment. I mean, what kind of a future would she have really had with him? (as if doing a comedy routine) She's got two jobs: Denny's waitress by day, Slayer by night. And Angel's always in front of the TV with a big blood belly, and he's dreaming of the glory days when Buffy still thought this whole 'creature of the night' routine was a big turn-on.
WILLOW: (disturbed) You've thought way too much about this.
XANDER: (excited) No, no! That's just the beginning. Have I told you the part where I fly into town in my private jet and take Buffy out for prime rib?
WILLOW: (as Buffy enters, trying to shut him up) Xander...
XANDER: (laughing) And she cries?
Relationships are personal issues, so yeah, I'd still say he's still in no position to disagree with them. Buffy and the rest aren't perfect when it comes to judging others either, but Xander's the one who does it most consistently...at least Buffy and the rest grow out of it after Cordelia. Anyway, I think disagreements cross over into judgments when he
judges their personal character. Which he does a lot.
Many, many occasions? I'd like examples, please.
I'm probably missing a lot because tbh I'm a little busy atm, but starting off with Prophecy Girl, after being (respectfully) turned down by Buffy, his first reaction is to mock her attraction to Angel. Right. Like, I get his animosity towards Angel, but to insult
Buffy for it just reeks of entitlement to me, as if Buffy's in the wrong for daring to have her own feelings. At this point she isn't even dating him either, so no, there's not really any pressing danger. Simply jealous, entitled behavior.
In Becoming:
Buffy: "What happened to Angel wasn't his fault."
Xander: "Yeah, but what happened to Ms. Calendar is. You can paint this any way you want, but the way I see it is that you wanna' forget all about Ms. Calendar's murder so you can get your boyfriend back."
It's clear that he disapproves of Angel throughout, but his comment about Jenny Calendar's death doesn't make much sense because out of all of them (except for maybe Buffy) he had the least connection to her, and so it just feels like he's objectifying her death to shame Buffy into killing Angel/Angelus. Not to mention Giles, who Jenny's death affected the MOST, is standing right there and even he didn't stoop so low. (This is really another thing I dislike about Xander- the way he throws a person's mistakes in their face like that to make a personal point. I mean, really? Geez, man.)
And Entropy, Xander shows again that he thinks he has the right to judge other people's sexual choices, and not just because those choices pose a danger to society:
XANDER: So you go out and bang the first body you can find? Dead or alive?
ANYA: Where do you get off judging me?
XANDER: When this is your solution to our problems; I hurt you, and you get me back. Very mature.
ANYA: No, the mature solution is for you to spend your whole life telling stupid, pointless jokes so that no one will notice that you are just a scared, insecure little boy!
XANDER: I'm not joking now. You let that evil, soulless thing touch you. You wanted me to feel something? Congratulations, it worked. I look at you - and I feel sick - 'cause you had sex with that.
Yeah...Can't say I'm a fan.