To deliberately create a child that will lack either a mother or a father is wrong. The child will always be missing something from lacking one of its parents and all you can do is mitigate this loss rather than fill it completely.
Having said that, having a child adopted by a single or a gay couple is a far superior alternative to having them grow up in a children's 'home'.
So to summarise, if you are a single or part of a gay couple, instead of massaging your ego by creating your own personal accessory that will be deprived of either a mother or a father by completely avoidable circumstances, go out and adopt a child that needs a better home than the one they are already living in.