Oh oh, oh oh. Then I am guilty that this subject does not die.

She lied, Mormont. When asked in bed "but you don't have a boyfriend?" and she came up with that little story, she lied. Remember our dear teachers' class? It's in the dictionary!
And this part of being afraid to tell about being with another girl was in class too, remember? She might as well have made it clear that she had someone in her life, without making it clear what sex that other person was.
Think for a moment. You come to a girl, somewhere, a party, a coffee, whatever. You start the conversation, clearly interested in her, then ask, "Do you have a boyfriend?" You are not interested in knowing which biological sex of such mate, you want to know is if she has someone! If she says no, since she has, she lied. But let's keep going... So you do everything with this girl, day after day, so someone knocks on your bedroom door, none other than this girl's girlfriend. What would you do and think about it?