Now the Bible does talk about one man and one woman in marriage. Is that not conclusive?

No, not really. The bible doesn't state clearly that marriage is between one man and one woman for life and anything else is sinful. There wouldn't be much debate on the topic if the bible had made it clear.

Also, the Bible talks about man not laying with man as with a woman. Wouldn't that take care of the homosexuality.

Only for those willing to accept all of the laws found in the rest of the book of the bible that passage comes from. If you consider it sinful to: wear clothes made out of more than one fabric, associate with a woman during her period, plant more than one type of seed in a field, trim your beard, or wear short sideburns, then that passage from Leviticus applies to you. If, on the other hand, you are ignoring all the Leviticus laws you find inconvenient, why would there be an exception for the one on homosexuality?

We do not follow the laws of the Old Testament. Thus if you want to consider homosexuality sinful, you need to show that it is still sinful according to the new law, the law of love spoken of in the New Testament. There are strong arguments in favor of homosexuality being sinful that rely on the passage in Romans, but there are also strong arguments against those arguments. Obviously some people are wrong and others are right, but either way, we are left with a very debatable topic.