That’s what netorare is: cuckolding.
Look, I get that it’s not a perfect match, but I think you have to ask yourself: who does this content appeal to. Even if you don’t make a move on Evelyn, you can still stop Thaddeus; I assume most players are going to take that option, unless they specifically have a cuckold fetish (or voyeurism fetish, if they aren’t romancing Evelyn). In my eyes, that choice is more of an option presented to the player than it is an in-game decision for Dickon’s character to make: something along the lines of, “Yes, I want to see Evelyn get fucked by another man,” vs. “No, I don’t want to see that.”
At any rate, netorare doesn’t need to feature an actual act of “stealing”; it just needs to feel like it does. (In a lot of NTR manga, the love interest gets stolen away even before the protagonist has a chance to ask her out or have sex, etc.) That’s why it’s so effective at triggering people (on this site especially): they feel like they have a right to all the girls, regardless of the reality of the situation. That Evelyn doesn’t seem to actually want Thaddeus, but is only acting the part because she feels she has to for diplomatic reasons, would only add to the outrage. (That’s the read I got anyway; I might be wrong.) Moreover, I believe that Evelyn being a love interest is reason enough to expect players—at least some number of them—to feel a degree of possessiveness towards her preemptively; especially so in a “pseudoharem” game like this—sleeping around with all the girls has no negative consequences at all before the very end, so there’s nothing to punish players for adopting a monopolistic, entitled mindset, or dissuade them from doing so.
