I cant tell you whats up with you and whether its normal for you but I can tell you how a male friend of mine who is heterosexual explained it to me...

he thought something was majorly wrong because suddenly he was fantasizing about sex with a guy... he went to his doctor. the doctor told him that a man has one of their most erotic spot (Male G Spot) by way of the back door called the prostate. most women dont think about doing whats needed to stimulate that erotic spot on a guy. to put it bluntly go up a guys behind. so whats left fantasizing about a guy or using sexual toys to stimulate their own G Spot.

made sense to me and Im a woman married to a woman. there are many things that we lesbians sometimes fantasize about, it doesnt mean we are heterosexual just means a need isnt getting met, or we enjoy experimentation and fantasy or both.