Its fine to have exclusive relationships. Its not fine to try to socially isolate your partner so that they have no social contact outside the relationship. I would describe such a relationship as abusive.

I strugle to interpret the grandparent's phrase "You want to be as exclusive as possible with the other person, so that you both have as much of other's attention as possible" in a way that doesn't support socially isolating your partner, which i would consider unhealthy as an ideal, and abusive in practise. All relationships take time and effort, whether that's friends or otherwise. If 100% of your attention (which is the max possible) is focused on your partner, it implies that 0% of your energy is focused on any other friendships/social contact. That is unhealthy. It should not be held as an ideal.

But to be very clear, you can still be in a healthy exclusive romantic healthy monogomous relationship, while still having other friends & acquaintances. Having other platonic friends is not the same as being polygamous/polyamorous/etc.