My parents won't let me sleep with my girlfriend?

I'm 20 and she's 21 and we've been dating for 6 months now. she's my first gf and i met her at uni. we're a very close couple and like lots of cuddles and things especially when in bed. i took her to meet my parents 5 months ago and they made us sleep in separate rooms. i thought this was ok cos they didn't know her, but as time went on we visited them a lot more and they really liked her. we left uni for summer today and we are living with them for 2 months but they still won't let us sleep together and we are both really upset. its not like we even want sex that much, we just wanna be together cos we love each other. i bought up the subject with my dad and he says we can't because then he'll have to start letting my 17 year old sister's bf stay and he thinks she's too young for that. i think this is unfair though considering i'm 4 years older and already live on my own. its just not like them to do something like this. me and my gf haven't spent any nights away from each other since getting together except when with my parents and its going to be so hard for us for 2 months. I'm just scared that i might start resenting them because of this which is a shame cos i have so much respect for them. what can i do?