Hi Rachel,
Since you got upset with your boyfriend, and he said he did not mean anything by it, he may be telling the truth. He may just see her as a good friend, and she brings out the playfulness in him. Next time, you and your boyfriend are together, watch a funny TV show, and show him you can be playful and corny/giggly too. Tickle him, tell him silly jokes, just something out of character. Show him you can be just as fun! Let him know you will not tolerate him flirting (which is what he was doing in front of you, which is disrespectful to you). If he continues, I would re-evaluate the relationship. Better yet, invite one of your male colleagues out with the two of you, and start asking him to go to the cinema with you, and tickle him. What comes around, goes around. Let your boyfriend know what it feels like. Another thing is not to let another woman on a date with the two of you. I have done this before, and it’s always led to misunderstandings and hurt feelings.