We love Leyala too! Happy that she's the one for you. No worries even if you choose to have a relationship with only her, you will have plenty of content.
By rejecting her, to begin with, you will only be rejecting lewd scenes. However, as the game progresses, if you are cold toward her, she will get colder too. We don't want to give away too much, but if you're nice to her even when rejecting her lewd scenes, you should be okay.
Other vampires won't say anything to your face for fear of losing their tongue or head. They would find it odd, but they know of the bond, so they will understand why you're doing that. They will just pity you because they will think that you must be bored to stay with the same person.
Well, Lucius was monogamous until what happened
So you would be the first to stay monogamous till the end. There will be a special ending if you chose that path.
You won't be an outsider, but an oddity in the eyes of your people.