Welcome to wikitree! This issue is covered under Non-Traditional Families you can read more about it here
There's nothing on WikiTree that requires a marriage to be between a man and a woman.
Even though WikiTree doesn't require traditional family structures, it does assume them in certain contexts. For example, if you add a spouse to a woman, the system will assume that his gender will be male. This is just the default. You can still make the woman the wife of another woman.
It's slightly trickier with parents. It is possible on WikiTree for a child to have two male parents or two female parents. This may be appropriate for living people if the child is marked as Non-Biological. However, for non-living people, genetic connections should be used when they're known.
Also note that having a man in the "mother" field or a woman in the "father" field won't look right in all instances.
Our database fields are limited, but the biography space is not. You should use the text section of profiles to explain non-traditional family structures and link to any parents or children that don't neatly fit in the family tree.