In the manga, the mangaka gives us a comedic moment where Aegir has a moment of vulnerability and calls Lucy 'mom', then he sweats bullets and she laughs and is ok with it. But in the source, there isn't a comedic moment here, there is a passionate moment where they're making love and it's Lucy who says something like, I'll be anything to you, your girlfriend, your mom, etc. To me, it came across not like an Oedipus thing but as her telling him how deep she wants their connection to be.
Also as someone else in comments talked about, Aegir is pretty much a barn child who knows nothing at all about the world, has no experience with life, and no one he is close to, so is it really surprising when he does something half retarded like this? Remember, we all saw him just shrug and not give any fucks a few chapters ago when his bandit bros kidnapped and gangraped a girl. He's not a very thoughtful kind of guy.