People grow and change, nothing is forever despite the romantic notions. Only you can say or know what goes through your mind and heart. Perhaps it's a "moon phase"; perhaps a life change. Only time will reveal the answer. Sorry if that doesn't help at the moment, It's just an opinion/observation. Even our "truths" change in life.
I think you are having bi-curious feelings, which is entirely natural to have regardless of whether one is straight or gay.
It's definitely not 'un-lesbian' to enjoy penetration. You and your girlfriend can use toys and strap-ons and fingers. The sex act itself doesn't define sexual orientation.
If you are starting to feel sexually attracted to men, then I think you're bi-curious and possibly on your way to being bisexual.
it's definitely something that you need to talk to your partner about if you're interested in exploring these feelings. You'll also have to figure out if she's willing to stay in a committed relationship with you and let you try out men. If she's not and it's become something that you want to explore then you have a very difficult decision to make on whether to suppress these urges and stay with her, or break up and be single where you can experiment with men.
I would encourage you not to get too hung up on definitions of lesbian, bi-curious, bisexual just yet. They are all just labels meant to box people into a certain definable sexuality. Sexuality is very fluid and changes throughout our lives. Attractions and urges can develop where they never existed before. It's completely normal. You just need to figure out if you want to (or are ready) to act on them now and what makes the most sense for you at this point in your life.
If you have an understanding partner and a relationship based on good communication, the first step would be sharing your feelings with her.
xx Olivia
Why worry about what other people think?
Reading your email I sense two separate concerns. One is your partner who you mention in the last sentence. Your much bigger concern seems to be that you might not be a 'proper lesbian' whatever that might be and what other people might think of that. Which is the sort of thing that did worry me at 19 but does not worry me these days.
The gay community can be rather insular. I am straight, but I write for a gay blog. Like many communities that has been formed by adversity there is a tendency for members to suggest that anything less than 100% commitment is some sort of betrayal.
There is another possibility here and that it is that this is being driven by your mating drive rather than your sex/bonding drive. If you are straight there is no real need to distinguish between the two. But there are good reasons to suspect they might be different.
We are products of evolution and in evolutionary terms a female-female pair bonding can be just as successful in terms of child rearing, provided that is there is a child to rear. And our genes haven't adapted to in-vitro fertilization yet, or for that matter the fact that we now live to 70s and 80s rather than 35 being old.
So don't discount the possibility that some bastard sexist gene in there somewhere is desperately trying to turn on the 'baby making machine' gene.
I must stress here that my argument is meant to be factual rather than normative. The fact that we are biological creatures with some multi million year old genetic code adapted to a particular way of life does not make that life 'better'. We are all of us living way outside our design parameters. We are also designed to eat and store as many calories as possible because we are adapted to the possibility of famine.
Understanding what might be going on is merely a tool to help find out a solution. Depending on you and your partner, that solution might run from having a date with a straight or bi guy[*], to her taking you with a strapon. It is also possible that she is having similar feelings.
I certainly don't think that there is any particular merit to being a 'gold star' lesbian. Better to try something and know than make an issue of not having tried it.
[*] Taking a gay guy to bed does not seem like a good plan in this particular circumstance. Not unless you want to make a comedic movie.
for the record, my same sex partner/gf/wife and i enjoy penetration sex and we're not fantasizing about guys while indulging - it simply feels good to have another woman making you, well... feel good. not matter what you're doing, she's still a woman.
You can’t truly call yourself peaceful unless you are capable of violence. If you’re not capable of violence, you’re not peaceful. You’re harmless.
Any insight would help! Thank you! 
