If their romantic significant others are fine being number two in this way, and it's all consensual, then they should do it and move their lives the way they want to.

I would personally not be OK with this setup with a romantic partner (neither as one who would see a friend as my number one or if the romantic partner did with someone else), but that's obviously not supposed to limit them in any way, since I'm not part of their equation.

All relationships could do well with clear boundaries, understanding, and a ton of communication to continuously maintain that as relationships evolve.