I'm 17, she's 14.

[Untouch]

Not sure about US laws, but here in the UK if a person over 16 has sex with someone under 16, it's automatically classed as rape as the other person is under the consenting age. Anyway, lets not get into that, don't want to worry the OP.

http://www.avert.org/age-of-consent.htm

Reasoning behind it:
Because the law views someone under the age of consent as not being mature enough to fully understand the consequences of sexual activity. If you can't understand the consequences, it's not possible to make an informed decision about it.

As a result, the law considers anyone under the age of consent as not being legally able to make a decision about sex. So, even if that person says "yes", it's still rape because it's a decision they're not empowered to make for themselves yet.