Just be there for her like you have already said daddylonglegs. She has probably known for a while that she is bisexual, or she may just want to explore her feelings with this girl if she looks up to her and wants to be like her. Either way, I imagine she came out with the 'you're homophobic' response as she is still young and probably is embarrassed about her parent mentioning sex - as any teenager usually is - and just wanted the conversation to stop there. She won't suddenly run off and start doing bad things, she has just fallen for a girl, just like most teenage girls fall for a boy.
Try and be relaxed about it, perhaps saying that they would be breaking the law came across as a threat, as one would assume by 14 she is well aware of this fact already. Sit back and be there if it all ends in tears, she'll be doing her GSCE's whilst her friend will be moving into new circles and getting a job soon so it may fizzle out, or it may work, either way just love her none the less :o)