She mades wedding invitations rather than cakes so she is making a very niche product. In the UK gay couples can now have a wedding rather than a civil partnership. She may have been confused since religious organisations are not obliged to carry out a wedding service for gay couples if they do not wish to, however this dispensation does not extend to commercial enterprises.
If she provides invitations for heterosexual couples regardless of their religious preferences or living arrangements then she has no reason to refuse to provide invitations for homosexual couples. She is discriminating merely on the basis of sexual orientation .