I mean, I don’t think he just being Beta. It feels more like he is manipulating her by making her be the one to chose. If she isn’t ok with it, then he had put it in a way that wouldn’t make her too mad. And if she accepts, it’s all her decision and she can’t cheat because the Mc told her how he wouldn’t like her being with another man. Over all the Mc made an Amazing move for getting what he wants while having low risks.
Well, atleast in connection to this Series, it had always been Sylphy who pushed MC towards other girls.
There are many reasons for it, like
- primary reason being, she needs MC's power which is useful for her goals
- there is also the reason of the bad gender-ratio between Women & Men in that World (think it was mentioned to be 7:3 or something) especially for the people of the Kingdom whose nation was taken over, and they lost a lot of people because of it.
- then there is also the fact that there are many Female Only races as well like the Harpies and those said Harpies are highly interested in MC, and Sylphy wants to make the Harpies work for her as they are quite valuable now because of MC's items and weapons, but they are also an autonomous group not connected to the Kingdom (they are just helping out as the nation which took over the Kingdom hates and hunts them down). So, she is willing to use even MC if she can get the corporation of the Harpies and many other people.
As for Sylphy going for another guy, so far there doesn't seem to be mention of polyandry culture, only polygyny is mentioned. Which is understandable considering the Gender-Ratio balance, and the fact that there isn't really much capabilities difference between men & women of other race (unlike human race, were women are generally weaker atleast combat aspect wise).
Plus, Sylphy being an ex-Royal doesn't allow her free time to even think about going for another man, when there isn't really anyone worth around.
She never really planned to have relationship with any man yet to begin with, primarily because Sylphy is quite young even for an Elf (by human age standards, i think it was mentioned that Sylphy is in her late-teens), and has a really long life ahead of her (Elves live for thousands of years in this Series, if i remember right).
The only reason she created a relationship with MC, is because he is really useful to her and her goals.
So, in order to reduce her guilt of using MC as a tool for her goals, as well as binding MC more to her.
She is allowing him to have relationship with any other girls he wants or any girl interested in him.