Disclaimer
*This is in no way an indictment of all women.
I prefer people in general to be upfront. The "I have a boyfriend," retort is basically the same as a not interested, but it changes when she lingers and makes a concentrated effort to continue the conversation.
Human nature is ugly, but the reality is *a lot of females, particularly in my age group,* will still entertain you if she's feeling you, even if she's in a relationship, especially those long distance relationships. Less so if the pairing still has that new car smell and hell - she might even actually be loyal - but she isn't giving you that line and walking away/deading the conversation unless she's repulsed by you or the guy she's dating really did find a diamond in the rough.
You can typically read body language and tell whether or not someone is genuinely into you. She'll give you that line so she won't feel like a complete hoe and can have somewhat of a clear conscience as she texts you later in the week and you schedule a meet up. Others won't say a thing about him at all, but when people are truly attracted to each other physically, the concept of right and wrong flies out of the window.