It's an amnesia arc.. SPOILERS
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They've literally been married for years at this point. And both are aware of that.
ML (the younger one) has been the one making passes at MC but MC kept refusing because ML is currently mentally a minor due to his amnesia.
Is it still questionable he gave in when he just turned 18? Sure. And if that bother you that's fine.
Just wanted to clear up some potential misunderstandings.

Amber’s been “18” for like 3 secs and that hornball alr has his dick out