Either the translators are messing with us for the lulz, this girl truly is a freak, there's something about Behemoth as it grows up that would make this actually reasonable that we are purposefully not being shown for the mangaka's amusement, or she actually knows who he is (or at least that he's a reincarnated human) and so doesn't see it as bestiality. I'm thinking she might actually be someone who knew and loved him in his past life who reincarnated and knows who he is.
Sure, I could read ahead or wiki or something, but I don't want to spoil it for myself and it's fun to brainstorm!
On the other hand, she didn't speak silently when confirming she wanted to get knocked up by him and while the guild master was surprised, he didn't seem bothered. So, maybe it's actually an acceptable, though unusual, thing in their society. Which would make some degree of sense for monsters who can crossbreed with them, I guess...