lol you poor guy that's harsh.
It sounds to me like your mum's worried you're going to "have pre marital relations" and that's why she's against it.
At 18 you're classed an adult and then it's deemed ok, not saying that's the case but I've known a couple religious families where that has been the rule.
In either case tell her you've asked this girl etc, you really like her and she makes you really happy. You'd have to be pretty self centered to deny your children of anything that makes them happy. Especially an innocent 14 y.o relationship, part of growing up.
Also try turning it on her if she says No.
Always ask for a good reason why not, even if she has one counter with "but she makes me so happy, why is that a bad thing would you rather I be miserable?" and go from there.
Good luck Wii-Fan!
13/08/2007
Super Mario World
Gold Coast, Queensland