For.

1. If you have a connection with someone worth to say you are in love, it has to be with one person or else it's not called love.
2. Love means giving every piece of you to the person you are in love with. How can you give every piece to ONE person if you're sharing with another one.
3. When you feel attracted to someone, you can be attracted to many other people but when you feel in love with someone, that emotion awakes for just one person.
4. When deciding between two things, even if we think we like both things equally, truth is, we always prefer one thing over the other one. So, if you think you're in love with two persons, probably you're just confused. You must be in love with one of that two persons.