For most anarchists, marriage and sex are private matters to be resolved between consenting adults. In their opinion, authorities like governments, lawmakers, churches, mullahs, etc., have no business meddling with private affairs of individuals who do not voluntarily desire to follow their commands -- at which point they are no more authorities and have no power to command those who dissent, only the opportunity to advise those who consent.

Note that, at least for most individualist anarchists and anarcho-capitalists, consenting adults having sexual relations do have to be held individually accountable for the consequences of their acts, whether desired or undesired, including any resulting children.

See also anarcho-feminism (aka anarchafeminism) for a combination of feminism and left-anarchism. Compare and oppose Wendy McElroy's individualist feminism.