What is jealousy?

Jealousy, in general, is an emotion that warns us when something we desire is being stolen, or when someone else has something that we want. In the case of relationships, I would say it is the former, also coupled with a fear of abandonment / a lack of affection from those we care about. Usually, people think that cheating partners don't care about them and sideline them, even if they care about that partner. It's unreciporicated because we think our partner's attention is elsewhere, hence jealousy.

That being said, whether people get jealous over this is ambiguous. Some people are perfectly happy with being in polyamorous relationships, while others get concerned if their partner wears leggings in public. I think the main factors here are socially entrenched values as to what makes a good partner, as well as that person's general level of security with themselves, and their trust in the other partner.

I think the best way to subvert jealousy is to prioritize the other person's emotions and talk to them about their boundaries. They should do the same thing with you, and you also should not try to control each other's behavior, as this can build tension in a relationship. I would never be with a person that didn't want me to hang out with friends because they look decent.

So the short answer is to talk about it with your partner and agree upon things that you think are ok/not ok.

"Do you think we should be happy for our partner if he / she is having sex with someone else?"

HELL NO!!!

I don't think there's a universal answer here

People can have good reasons to do one thing or the other and like in this case it's hard to actually claim what's objectively better

If you're with someone who gets jealous over that then you have to decide what's more important, your wants or their wants

What's "right" is understanding each other's perspectives and being able to facilitate compromises and recognizing if something will fundamentally work or not

If you get jealous over the gender of a partner's friends and they just want lots of different friends, then you need to accept it's not their responsibility to change their choices to suit the relationship to your standards, but rather recognize that you two are incompatible

Thanks a lot for your answers.
So if I understand the feeling of jealousy is unique to everyone. there is no single definition.
Does that mean that if we want to get into a relationship with someone we must first talk about what we can appreciate and what we can not like about our behavior?