answer 1 Edit
The Catholic church requires a marriage to be consummated before it is valid, which means "yes". In certain locations this is or has been a legal requirement. On the other hand, modern laws proscribe stiff penalties for forcing your spouse to have sex with you, considering this intramarital rape. If you live in a country with a law like that, you cannot be forced to have sex even if you get married.
answer 2 Edit
You don't have to, but it would be nice :). If both husband and wife agree, it's perfectly ok not to. But if one of them doesn't agree to this, then you would be putting a lot of stress, pain and suffering on the one you love.
hope this helps.
answer 3 Edit
On a less dogmatic/biased perspective, marriage makes sense only as a reproductive support association between two individuals. Otherwise, it's as much meaningful as unmarried couples. Hence, same-sex marriage being nonsensical due to the inability to reproduce. Current society standards will have to be updated to draw the border between an economical/legal union and a "marriage" reproductive effort.
Ultimately, there is no obligation for actual sex, there is only the need to reproduce.
The truth is never beautiful, and few are able to face it.
answer 4 Edit
This is a topic each couple has to answer for themselves; as the first answer says, anything can work if both people agree.
Most people enjoy sex, and for many couples it adds an important dimension of intimacy to their marriage. Some people who are afraid of sex or have had bad experiences with it can learn (with patience and understanding) to enjoy it as well.
However, some people really do want companionship without a sexual relationship. You can learn more about this at http://www.asexuality.org/ .