I just finished the bonus series on Nar Shaddaa with my female agent, and she slept with the colonel who starts it. So male and female characters both can choose to have a one-night stand (or possibly more, the way the colonel describes it sounds more like a long weekend) there. It might be solely agents, though it would seriously surprise me if bounty hunters at least don't get the same treatment.

The one gap between flirt options that I know of is with the person who gives you flashpoint quests in each faction. Male smugglers -- only smugglers, no other class -- can flirt with Satele Shan, though it's extremely one-sided, since she just keeps saying no. No one can flirt with Darth Malgus.