As today's bride and groom celebrate their wedding, they have every excuse for being nervous.Although Britain is still conservative in its attitudes to marriage compared with other countries such as the USA, Sweden and Denmark, the future will probably see many more people living together before marriage - and more divorce.They exchange promises of lifelong fidelity and mutual support.Not only are there more and more divorces, but the number of marriages is falling.Their own marriage has a one in three chance of divorce, if present tendencies continue.Traditional marriage is facing a crisis, at least in Britain.Living together is more popular than before.