It can be a factor of a person being uncomfortable with your life choices, media portrayal of bi women and also a bit of ignorance and fear of being possibly attracted to the same sex.
I am a very straight woman that has a lesbian friend, a bi friend and an on and off bi friend. I have had all three women let me know in no uncertain wording they would 'do me' if I was so inclined. I let them know that the compliment was accepted but the interest is not there. The subject was dropped and our friendships are still intact. Two of the times this happened, my friends were drunk and a lot more open about their lifestyle.
I was not threatened, but was a bit confused by why they brought it up knowing full well I have no attraction towards females--the one I was comfortable asking told me she just wanted to let me know in case I ever thought of reevaluating my preferences....she was not trying to convert me or hit on me. Just let me know that there was an option if I ever considered it.
One thing to remember--women have closer personal relationships and things can get misconstrued. Your friendship could be threatening to a straight woman who is not confident in her sexuality, who thinks that 'bi people are attracted to everybody' instead of the reality, and are ignorant of the fact that you can be friends with bi/lesbians and not have anything sexually in common. Just a good, solid and close friendship. I have gotten flack about being friends with my lesbian friend--but let me tell you 31 years of friendship is not worth tossing to the side over bullshit rumors.