I have to be married.

Untrue

Single people, including single men, can adopt. Also couples in a permanent relationship but who are not married can adopt, although not jointly. This however is likely to change when the new adoption law comes in. At the moment children's charities are campaigning to allow all un-married couples the same rights as married couples.

This can have certain legal implications but does not necessarily affect your parental rights over a child.
For more information on adopting as an unmarried couples.