Does direct sex discrimination apply in cases of women only?

No. Under the Equality Act 2010, direct sex discrimination occurs when, because of the protected characteristic of sex, an employer treats a woman less favourably than it treats, or would treat a man. The treatment does not have to be because of the woman's actual or perceived sex: it can also relate to the actual or perceived sex of another person (for example, a person with whom she associates). With the exception of special protection afforded to women in relation to pregnancy and maternity, the provisions of the Act apply equally to women and men, so that a man may complain of direct sex discrimination where he is treated less favourably because of sex than his employer treats, or would treat a woman.