I was a single lesbian who had a profile on a GLBT site when a married woman took an interest and wrote to me and told me why my profile caught her interest. She told me up front that she was happily married, bi-curious, and had no intention of ever leaving her husband. I don't think she knew at the time whether she wanted a separate relationship with another woman or if she wanted to include the relationship to be with her husband as well. I wasn't interested, but thought that we could still become friends. We continued to write each other and explored our thoughts and feelings. Everything just kind of developed and grew from there.
With her husband's permission, we eventually met and even messed around a bit, but she was feeling guilty about not having her husband included. I eventually opened up to the idea of the 3 of us getting together and we worked on that slowly. She has an amazing husband and family and I am glad to have met them. I love them both, but in different ways. We have had our fair share of ups and downs throughout this relationship for over 6 years. We used to live an hour apart, but now they live practically across the country, so I am not sure what lies in our future for now. I don' t think any of us knew what polyamory was at the start of this relationship.
The point I wanted to make is that labels don't always tell the whole story. Especially if one is still trying to develop, explore or broaden their ideas about what they are looking for in relationships. It was easier for me to identify as a lesbian because I preferred to date women even though I have always been open to the idea of dating men as well. I just don't seem to get attached to men they way I do with women. I always grumble when it comes to labels because I always feel like there is an asterisk involved. Explanations are not always easy, so I just grabbed whatever label was easiest for me to get my point across and figure I'll do the explaining later if someone was interested in hearing me out.
I don't see anything wrong with using GLBT sites (or any other sites for that matter) to possibly meet people. If someone interests you, I think what it comes down to is just letting them know what attracted you to their profile in the first place and then letting them know who you are and be up front about your situation and what you are looking for.