Grace, I guess I'm in the don't worry about it camp since it was likely shared without any ill intent.
Personally I like to avoid terms like micro aggression because it can end up putting the other person in the position of having to defend themselves on something that they never thought about or intended (and yeah I know the idea is supposed to be that the person must be subconsciously thinking bad things, but is it really fair to assume that?).
I would say that, if you have a good relationship and it's something that bothers you, share from your heart what you feel with them (recognizing that they cannot anticipate that the feelings evoked in you by what they shared - that's why you have to tell them), giving them an opportunity to understand you at a deeper level and become more sensitive to you.
I suppose it all boils down to is when we are different sometimes others just can't stop thinking about how we're different and get sort of OCD-like in talking about things that accent our differences. I mean, we can talk about such things when the signs are aligned to have a meaningful and mutually beneficial discussion, but we sure don't want to be talking about transness or cisness or whatever all the freaking time!
Love,
Denali