Is my Best Friend a lesbian or not?

My best friend gets really annoyed when I call her a lesbian, or a "half lesbian", I dont feel she fits the criteria to be called a "bi-sexual".... She started foolin around with another good friend of ours, 2 YEARS ago! Yet she lives with her boyfriend... I believe it started out innocent, but very soon after she started playing with one of our friends, she stopped having sex all together with her man!
Last summer I started to question her about her relationship with the two, and she exclaimed she was just having fun... Its now been over two years and the relationship between my 2 girl friends has become as intense as any man and woman...
She then came to me a few months back and found out she was pregnant and claimed she didnt know how it happened cause she never had sex with anyone except her girlfriend.. She claims she went to sleep one nite and as always her man came to bed to try and get some, and this one time he got alittle further than normal only cuz she was asleep.. it supposedly lasted 2 seconds, yet she got pregnant!I asked her whasst she was going to do and shes going to keep it, and has discussed marraige with her boyfriend and yet she is still with her girlfriend on the side...
what worries me is I overheard her conversation with her girlfriend last nite and I heard her girlfriend refer to her unborn child as her baby... and they plan to ask eachother to marry on christmas.
So am I crazy for believing shes a lesnian