there are 3 seperate drivers at play here

1: Desire
2: Romantic love
3: attachment

You can desire one person
Be romanticly in love with another person
while also maintaining a supportive loving committed relationship with a third person.

They are 3 different things and most people are capable of having all 3 at the same time. The existence of one does not diminish the feelings you have in either of the other two.

If people understood this, there would be a lot less drama in the world.