How to deal with my girlfriend's family?

I don't know what to do.
A few months ago, my girlfriend and I were caught doing it for the first time. With the attention brought to her parents, they were obviously shocked and didn't approve. Anyways, ever since her whole family has been giving her a hard time, insulting her, putting her down ect. Just last night for example, her father boast out that, 'pretty soon, he's (me) going to have to pay fees,' implying that she's a hooka.
Her whole family is lowering her self esteem, and then she goes back and cries to me about it.
The problem is the more she sees me, the more her family does it, and then the more she comes back to me. It's a repeating cycle.
Another problem is I don't know how to deal with her mother. She doesn't want us seeing each other, and in result, every time she sees me, she tells me exactly what's on her mind. I don't know how to deal with this, because with almost every thing she says, something instantly comes to my mind that I could say back, but I don't because, any smart ars remarks would obviously make the situation worse, and any serious arguments involved with real things that are relevant to the situation, if made would only get my girlfriend in trouble.
What do I do?

Well here is what I would do. I would get her family together.. and have her and you there. Then you tell them how you truely feel about her ( hopfully you say you love her.. ) Then say that she's not a hooker or anymore insulting things. Also tell them they should of suportted her unstead of hurting her and that you would never hurt her like that. Then you should say that it was the first time.. to make it clear.. and if they havn't thrown you out of the house yet.. then they might of got somthing.

here_4_us,

I would say you should be able to sit down with her parents, as horrifying as that sounds, and talk to them in a mature way. Saying (basically) 'Hey, I love your daughter and yes, we have been having sex, and no, I don't think it makes her a bad person' may sound like a hard thing to do, but in the long run, it is your relationship with her and if sex was a mutual decision, then there shouldn't be a problem with it. Her parents may not realize just how differently they have been treating her and with this conversation they may be able to set their relationship with her back on track. Also, encourage your girlfriend to stand up for herself. I'm assuming since you are having sex that she considers herself mature enough to talk to her parents and let them know what's up, and what she doesn't want to hear (ie. 'you're a slut').

Good luck!

I agree with the 2 above talk to the parents and be calm no matter how hard that may be. they will most likely respect that and if they yell just stay calm and it may get them a little annoyed and cause them to yell more but just take it and sooner or later itll stop ^ ^... its going to be hard but itll gain their respect

the same has happend too me & my boyfriend.
my mom's boyfriend came home early & caught us. my mom wouldnt talk to me
& she called me a slut & slapped me. but the more my boyfriend
talked to my parents the more they started too see how much I like
him & how much he likes me. all you got to do is play it off. be cool with
her dad. my boyfriend texted my mom & talked to her about it.
& they dont talk much; but they are cool. & my moms boyfriend; well he is cool
enough to joke around.
like my boyfriend was saying sumthing bout 'best time of his life'. & he said
sumthing bout Riding Buggys; & my moms boyfriend was like 'You hear that
taylor; that thing yall did wasnt the best time of his life'. ha. wow. my
reaction to that was like what the f*ck? lol. but I hope my advice helps you hunn'.

Thankyou all for this very helpful advice.
The problem is, her mum isn't cooperative when she talks to me, so I don't think that I could arrange something like that, not to mention her dad's a huge bouncer, who has been known to get out of control, and wants to run me down in his four wheeled drive.
I managed to get my mother to talk with her mum, to whom she cooperated with much more. But my mum did not know the right things to say, so that only went so far.
Next time I'm confronted by one of her parents, I'll try to remember this and make an attempt to explain.