There are levels of love. The lowest of love would be that which is unconditional for all living creatures, the love for them for many reasons such as loving them even if they are not a good person could lead to them becoming better. Love for them because it is the right thing to do, to love everyone. While I feel everyone should be loved this does not mean crimes should go without punishment. This low form of love is necessary, I feel. But can such a strong emotion as love truly ever be considered "low?"
Then, what about romantic love? To long for someone, to want to be with them at all times, to have a desire, a hunger for that person. To risk everything for them, to defend them, to stand by them, to watch and guard them. Love is to be one with that person, for if anything happen to them the pain they bear will be with you as well. If the person you love were to die then you would also be dead, inside, at least a part of you would be, for they were that part of you.
Can or should you romantically love only one person? I don't think so. If you happen to fall for more than one person why should you be bound by a choice? Because society decides so? No, there is no problem in loving more than one. "Cheating" is the hiding of physical or emotional connection to someone other than your significant other, not loving someone else.