I'm in the early stages of a relationship - not sure if it will go very far at present, what with us being in different cities for the summer and it's a case of "we'll see how things go when we're back at uni". I don't regard myself as young, free and single anymore though.
As far as flirting goes, I'll admit that I have been guilty of flirting with a couple of guys - mainly just banter, and if I get the feeling it's going too far (ie. I'm asked for my number etc) I will back off and use the "sorry, I've got a boyfriend" line.
I think it's a case of boundaries... so long as it's just a bit of banter and good natured flirting that goes no further, and so long as it's not hurting anyone, then there's not much harm in it.
Perhaps if I were in a more serious relationship, I'd feel differently about it and not feel the need for the 'ego boost' that comes with being asked for my number etc. But I'm a person without a whole lot of self confidence... and male attention makes any girl feel good about herself.