It doesn't always contradict a preceding statement. It can also mean 'in view or consideration of the fact that'. It has been so used to begin a sentence since at least the fifteenth century:

Where as þe seyd William Paston, by assignement and commaundement of þe seyd Duk of Norffolk..was þe styward of þe seyd Duc of Norffolk.

Whereas has two distinct uses. In legal contexts, it means 'in view or consideration of the fact that'. It has been so used to begin a sentence since at least the fifteenth century:

Where as þe seyd William Paston, by assignement and commaundement of þe seyd Duk of Norffolk..was þe styward of þe seyd Duc of Norffolk.

Its more common use is as a comparative or contrastive conjunction. What others have said about starting a sentence with conjunctions such as but and and is quite right. Doing so is a normal part of a writer’s repertoire, and there should really be no need to discuss it further. Whereas is a little different, because its meaning makes it an unlikely sentence opener. A sentence such as She went on to become an architect, whereas I did history would not normally be split in two to become She went on to become an architect. Whereas I did history. But if a writer thinks that will achieve an intended communicative purpose in a given context, then there is absolutely no reason not to do so.