This is also a character role/dynamic thing. If you establish a character as a love interest (like Lois Lane) and aren't planning a love triangle as part of your main plot, readers generally will only accept the first love interest unless the first love interest is completely removed from the storyline (think like Gwen Stacey/Mary Jane Watson).
The big downside too with multiple love interests, is that authors make the differences between love interests too stark so we prefer one to another. Like how many stories have a girl unhappy with the Boy A, only to run to Boy B. Boy B is the better fit but Boy A "changes" so Girl comes back to him. If the story is a drama, Boy B then goes through an uncharacteristic personality change so Boy A is now the better choice.