Taken from in-game:Because he'd taken over her palace in the eyes of the world and kept her as his fuck toy which is what people think.
Sounds to me like she just wanted to make sure it was safe to broke a deal not offer anyone to a harem.
I'm not taking anything at face value, too many things can twist here and taking the princess that isn't going to be leading makes absolutely no sense, she'd be useless.
SE (Stenai Envoy):Queen Janine... it is a relief to finaly be standing in front of you.
J (Janine): It seems you've came a long way under duress. What message do you bring?
SE: First I've been instructed to ask... has the Doom King been gentle to you personally? He has not been... unnecessarily violent?
J: ... the Doom King has not touch me personally, despite what rumors say.
SE: I see. Well, I'm here on behalf of the King of Stenai... he would like to surrender his kingdom to you.
J: What?
M (Megail): Well, that doesn't make much sense. Due to you being on a different continent and all.
M: Plus, I can't imagine Ghenalon or Erosia would be happy about you inviting us in. Why are you really here?
SE: We are entirely serious. Stenai is constantly threatened by more powerful neighbors... we would be safer as a vassal.
J: Then why not join Ghenalon? That would be obvious solution, geographically.
SE: The King is not convinced that even Ghenalon is safe. Not with two Incubus Kings on the continent.
SE: We have heard rumors of a conflict between the Doom King and the Fucklord. We could... offer assistance with an attack.
M: Not military assistance I assume. Magical?
SE: Our mages estimate it is currently impossible for you to teleport into Erosia. And you are unlikely to get permission from anyone else.
J: Ah. So you would offer us a foothold on Thenours in return for protection... but is that the entire deal?
SE: We hope that you would help stabilize our realm as well.
SE: The King have no heirs, so matter of succession becomes complex. We would prefer to avoid such.
M: Understandable, given how much trouble a succession crisis can involve.
SE: There are currently two princesses from royal families. If the Doom King were to marry one, it would seal his ownership and also resolve our problems.
J: You would marry them away to save Stenai?
SE: Princess Reletima and Princess Feremina are both willing to sacrifice for our nation, and their families.
J: ...
M: Look, we're not coming tomorrow or anything. But it does look like we're at war with Erosia, so we'll seriously consider your offer.
SE: That is all I could ask. I will remain here to set up a channel to Stenai if you so choose.
Basically, in order to avoid succession crisis, they are giving away a Princess. Calling it a marriage is nice, but as Aka later points out, for all they know they might be selling them into sexual slavery. Janine agrees with Aka but also point out that they are aware of that and are still making the offer (they might even expect it, hence the question about DK not being "unnecessarily violent"). Simon might try to avoid having one of them "marry" DK but somehow I doubt such offer will be accepted... like the envoy said, they want protection and stability and you can't have stability with unclear succession.