I think most people make "interesting" faces, and/or do other unique things at orgasm. And most of our partners actually find them a turn-on, if only in a "charmingly cute" sort of way. What many (probably including your G/F) don't realize is that most of us are emotionally naked and vulnerable at that moment. Early in our marriage I made the mistake of telling my wife how much it excited me to hear the cooing/mewing/whimpering noises she made as she approached orgasm. No criticism - no sarcasm - just simple appreciation. But it made her self-conscious, and she clammed up - it was probably 4 or 5 YEARS before I ever heard those loving sounds again!