User blog comment:Read Books/Wondering?/@comment-7340329-20141101213509/@comment-25156300-20141102010456

But is it ever specified that they only mate within the clan? I know some cultures have no problem with it, but I've never been led to believe that the Narutoverse does that. That's why I find it so weird when OC's in a canon clan love a canon member of the same clan (usually Uchiha OC's with Sasuke).

Back to my point, notice that some members (in any clan) look different from the standard appearance of the clan (Hinata and her mom, a few random uchihas, etc).

I see it as they mate with a non-member and have them join the clan. I've noticed that we never see both parents in a clan using their kekkei genkai, only just the father or just the mother (more often father). If this is the case, then they would use their strict rules to make sure the new mate stays loyal to the clan. This would likely include preventing them from being involved in clan specific business like learning clan secrets.

Taking this into account, The possibility of a 'one night stand' type of deal is more plausible. For this specific case, the father would need to keep it completely to himself (or not know she got pregnant), so Itachi/Obito wouldn't know of it.