a true sikh is one has been baptised by the panj (recieved "amrit"), see earlier topics for more info. and i will answer your questions for a amritdhari sikh:
Does either family or the woman or man pay a dowry?
No, we believe in equality. i actually recall a hymn written by the guru praising women!
Age allowed to marry?
there isnt an age defined, but should be of a suitable age, where the bride and groom are mature enough to make this life commitment.
Engagements?
religiously there it isnt a must for an engament party, but for many there is an engagement and celebrations due to indian traditions.
Is Pre-Martial Sex Allowed?
No
How is Marriage defined (man and man, woman and woman, man and woman)?
man and women
Is marriage required?
yes we believe we can be liberated while living a family life and sharing with others, while doing as many prayers and mediation on god as time allows us. we don't believe like some eastern religions to go to jungles (or not get married) and JUST meditate on god. so family life is important, and this is the natural order of god for us to have children.
Is divorce okay?
No, but due to extreme circumstances it is understandable why some do get divorced. its not like you'll go to hell or suffer punishments.