Living With A Bi-sexual Partner.

I am a 22 year old lesbian, i came out 4 years ago and have a daughter of 5 years old. I always swore i would never get into a relationship with a bi-sexual woman because i didnt want to feel inadequate, but a year and a half ago i fell in love with my current girlfriend, who is bi-sexual. She has never had a serious relationship with a guy and has been in two long term relationships with women. She has always reassured me that i have no worries about her leaving me for a guy, but there's a few things that have made me paranoid. We were talking a few months ago about having a child together, and when i mentioned the cost and finding a donor, she seemed to think that we would do it by her actually havinig sexual intercourse with a guy and was dissappointed that i was naturally uncomfortable with this. She has also mentioned that she has fantasies of me and her having a threesome with a guy, which i suppose is to be expected from a bisexual person, but i have just been on the history on the computer and lastnight she was watching threesomes on a porn site.. none of which involved more than one woman. It is starting to make me feel inadequate and i dont quite know how to deal with it. I am madly in lovve with her and we have a lovely house and life together, but i cant help think that she wants sex with men. I have even been considdering letting her do it just to get it out of her system. Some advice would be greatly appreciated. Thanks.