The House of Lords is appointed, not elected, and so is made of a strange but quite brilliant mix of old politicians, aristocrats, experts in various fields, and even Bishops. It is a jumble of society, with appointments being made by the Prime Minister and a committee that recommends people to him.
Therefore, it is a place where all governments have had their problems, since the government usually comes out of whoever has the majority in the Commons, not the Lords. This government, in particular, has been met with defeat at the hands of a House that does not have any majority, due to the large number of cross bench members.
The Gay Marriage Bill is going to come under scrutiny, since the role of the second house is to make sure that all legislation is properly considered, amended and well written, something that you don't have to be that involved in politics to realise that it isn't. In fact, this could be the government's biggest defeat in the second House yet, although they have suffered so many that it would be a bit of a generalisation to say that.
If the Bill is defeated, it will likely cause a headache for Cameron, who is looking to get this things passed as soon as possible, so he can reap the electoral benefits he is promising himself, but also so that Conservatives stop looking at him so funny, since he is clearly not being that Conservative. This is something that might actually become a reality, as many regard the vote as being to close to call, and it is something a lot of people will be eagerly awaiting the results of.
The House cannot block the Bill for good, but it does at least have the ability to block it for a time, to have politicians really think about what they are doing, and possibly even amend it so they can make the best of a bad job. Gay Marriage is something that seems to be a done deal, but the negotiations are still ongoing for the details of what that deal is.
It could be that a lot of the problems with the Bill are going to be rectified due to the House of Lords, and that can only be a good thing. It will be the second House that it is meant to be, protecting the Commons from the slip ups that democracy is so determined to make.
Gay marriage is not a good idea, and is likely to have a very negative effect on society, but right now there is the opportunity for a lot of padding to be put on the Bill to protect society, and it is an opportunity that we cannot afford.
Then again, God is the God of miracles, so things might turn out completely differently anyway. Only time will tell.