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.
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.