Yeah, very few characters are monogamous in the game and nobody cares if the Champion is or isn't. You can date/marry as many characters as you want without any kind of gameplay or story penalty (it may affect potential ending variations, but Savin only knows that) and if you do some of the events like Atani's marriage, the fact that neither party expects it to mean vows of monogamy comes up. Now, if you do want to play that card you're free to but the writing isn't going to account for it so it's one of those mental roleplay things.
As for unlocking more date options, there aren't a huge number in the game yet but more will be coming. Ryn has date scenes, but only in the Winter City and only after you've cleared the Catacombs.