Okay, that seems... ominous. I'm hoping its just his personality, and not something that either automatically happens or is part of a process that is done to the Desmond children as they get older.
I know, dramatic reveal intended for the audience, but dude: If you want to tell someone what you're wearing, tell your allies after the enemy is dead or fled.
I assume that, as proper young ladies, you will be wearing dresses while courting, and , as such, abs will A) not be in evidence until the wedding night and 2) Are probably not grounds for an annulment.