Depends on what kind of lie.
I'm just thinking out loud here, but the distinction is how much her (or his!) decision to sleep with you is based on that one lie in question. If you're lying to get them to like you better in a more general sense - i.e. you lie about your favorite color, what food you like, your grades in high school, or whatever - then, while not exactly admirable, these aren't really serious lies. Who hasn't downplayed their hate for a food their love interest enjoys, or listened with interest while they talked about something less than interesting, or watched Twilight and pretended to think it's deep and meaningful? Most have, and not much comes of the lie. White lies function as a social lubricant; if you don't need it, great, but most people do, or else they don't think about it and just lie without even noticing. So long as your whole relationship isn't built mostly or entirely on these lies, a fib here and there isn't going to do much harm.
If you're lying such that they're sleeping with you because of the lie (or they're not refusing to sleep with you because of the lie), then it's more problematic. Denying you have an STD that you have, or perhaps even claiming to be wealthy when you still live in your mother's basement, gives them radically false information which robs them of the ability to make truly *informed* consent (informed as to the consequences of their having sex with you - they think they'll be healthy and potentially rich afterwards, which they won't). Of course, women (and men!) shouldn't be silly enough to believe men (or women!) who make these kinds of claims, but that doesn't excuse the liars from lying in the first place.
Between "Yeah, blue is my favorite color too!" and "AIDS? Me? Never!", there's a really big grey area. Hard thing to give a point-blank answer to - and I don't think your intentions or feelings towards the person matter all that much.
