Seen as wrong according to natural moral law and so it is forbidden as a secondary precept. If the categorical imperative states that sex is permissible as long its in marriage then that would not rule out contraception according to Kant. You may question if all forms are acceptable as some might cause the termination of an embryo and that may be considered as using someone as a means to an end, though here you could question whether an embryo is an end according to Kant. Utilitarianism would be fine as long as the consequences are good. Virtue ethics may consider it as being virtuous not to bring unwanted children into the world.