RSP homework? or just curious? anyway - can't help with the islamic views, but Roman Catholics believe it is sinful, most other denominations of christianity do not neccesarily agree with it, but they accept it as long as the partners are committed to each other, and faithful. Many believe that it is better to get to know somebody by living with them before marriage, than find you are completely incompatible as soon as you get back from the honeymoon...
Although everyones views are different, it is important to remember that most strictly religious people do not believe in sex before marriage (basically, cohabitation) and also that all cohabiting couples should be planning to marry eventually - especially before children some into the equation. Hope thats helpful, its about all i can remember from my GCSE RSP last year! C