...There are people who are capable of being madly in love with two people at the same time while giving each of them equal time and attention, but I would think that would be more rare.
My nesting partner (8+ years r'ship) has had a bf for 4 years. She is his only partner. She doesn't give us "equal time" because he doesn't WANT that much time together. He is introverted and for a long time was content with 2 overnights a month. Then he took a year off from her because he felt too crowded with that. Then he got back in touch and since then, they have gradually increased to one evening date and one overnight a week, plus messaging several evenings a week for an hour or two.