First of all, your daughter's sexuality is nobody's business but her own - this includes you. It is great that she felt comfortable telling you... hopefully your reaction hasn't damaged your relationship with her.
I understand being worried about disease - but simply being a lesbian does not mean that the other woman carries disease. Anyone (male/female, straight/bi/gay) can carry an STD. Here is some info on STD's (INCLUDING STD's and lesbians) http://wso.williams.edu/orgs/peerh/sex/std/
Your daughter's soul is between her and her god. It has nothing to do with you - much as her sexuality has nothing to do with you. Both issues are hers alone to come to terms with.
Your efforts to thwart your daughter's relationship with the older woman are only going to push your daughter away. You have said that she said she was confused. If you keep trying to push your beliefs on her, you will surely push her into the arms of the older woman. Try supporting her - you can support her and accept her regardless of your personal beliefs. Give her time and space to figure out her sexuality. If she ends up deciding that she wants to be a lesbian - then accept and love your daughter - WHO she is does not change because she decides to be intimate with women instead of men.
There is a support group for Parents, Families and Friends of Gay and Lesbian people. I think this would be a good resource for you - they have a lot of information that will help correct your misconceptions about Lesbianism and can also help you cope with the bombshell your daughter dropped.
It's called PFLAG.
Here is their site: http://www.pflag.org/
They also have a page of Do's and Don't's for when a loved one "comes out" to you... you might find this useful....
May you find peace and love in your heart for your daughter - regardless of how she chooses to live HER life.
The information about the several affairs was provided AFTER I responded and would have been helpful to know up front. it does make your concerns a bit more understandable. Knowing that doesn't really change my response - except to say that perhaps you could pass along the STD info (and how to protect herself in a lesbian relationship) to your daughter. Just because you know about STD's doesn't mean your daughter knows everything there is to know - and being new to lesbianism - she may not realize some of the ways the diseases are contracted.
I don't understand how she is losing her identity - it seems to me she is actually asserting her identity and figuring out who she really is... not losing it. I went through something similar (though I wasn't a lesbian) and ended up figuring out that I preferred men - so I can somewhat relate to your daughter's experimentation.