Seeing that your daughter is 5, I really don't see this behavior as anything more than experimentation. I'm sure her witnessing her mother kissing other women affects the gender she chooses to experiment with. However, I really don't believe this has anything to do with her sexuality. If your daughter is witnessing significant sexualized behavior from her mother, you might want to have a talk with her mother as I view this as inappropriate in the presence of young children.
I'm sure that it is most likely just experimentation because she is a curious child. Though she has shown a lot of interest in her mother's sexuality and I know that a child could inherit the so called 'gay gene'. And from information which I've looked up on on the internet I've seen that 5 is a common age for a lot of children to notice that they are 'different'..
Me and the mother have talked about how we behave with our respected partners around her but I don't believe it be fair for me to be allowed to kiss my partner. While she isn't allowed to purely because her partner is of the same sex.