There's a lot to unpack here...
Firstly, different cultures have a massive impact on social values. Even more so when death and murder are so normalized within those societies.
Attributing your cushioned modern societal values onto a Githyanki, who was literally raised in a kill or be killed society that seeks strength above all and deems anyone who you are capable of killing to be so inferior as to not warrant attention is misguided.
Human history has plenty of behaviours that our modern values would consider shocking based on the societal norms of the time (For example, King Henry VIII beheaded 2 of his wives for the mere act of... Birthing a girl instead of a boy). Let alone quite literal alien societies, in a fantasy setting...
Secondly, different cultures and different individuals have different notions of relationship values. For example, Githyanki (And as a result, Lae'zel) do not care much about relationships. They aren't necessary for their society (Given the way they reproduce with designated egg layers), they're merely a means to an end of having sex for fun. Lae'zel soften up somewhat over her romance arc, but she's still going to be dealing with her Githyanki values.
Thirdly, beliefs are very strong. Even in the real world, there are people who hold their beliefs above all else, including their loved ones. Not everyone is capable of suddenly ditching everything they believe in because they fell in love with someone. So going against someone's beliefs won't naturally just cause them to jump to your defence.
Fourthly... The overall game of BG3 takes place over a short period of time. Like even if you long rest gratuitously you're talking about a month total (And if not like a week or so). How deep a relationship will you form over that short period of time? You are trying to compare this short a relationship to people who've been together for 10, 20, 50+ years, where there is a much stronger bond and a lot more loyalty between them.
Fifthly, people enjoy this sort of thing because it gives characters depth. They have convictions and beliefs, allowing them to be individuals. They don't just suddenly become your own "Yes man" because you slept with them and therefore will never ever disagree with anything you ever do.
Heck, trying to spout off that a partner can never be wronged is how relationships are, shows a complete and total ignorance of relationships. Where it's in fact toxic when partners stop thinking for themselves and will completely abandon their own beliefs to appease their partner (This is actually a major problem and is part of why it's so common to find people stuck in abusive relationships, because they put their partner above their own self and values so they continue to stick around receiving abuse)
Sixthly, Lae'zel threatening to kill you after sparing the Emperor isn't just "A wife threatening to kill you because she doesn't get what she wants". You aided a Mindflayer. The sworn enemy of her people. A race that uses deception and manipulation to further their own ends. This particular one has also been keeping the saviour of her race locked away, killing countless Gith who tried to save that saviour, all to utilize this saviour for their own selfish purposes.
To her, this is a MASSIVE betrayal. You call her unloyal for threatening you... Yet disregard your lack of loyalty when you completely and totally went against her entire beliefs and helped the one being she hates the most in the universe. Even if she wasn't from a aggressive and bloodthirsty society, that'd get most people to turn on you.
