It depends on what the birth control is and what it is being used for. If it is for example birth control pills that are being used to treat a medical condition then it is not sinful. However, if it is artificial birth control pills being used for the purpose of preventing pregnancy then it is grave matter that has the potential to be a mortal sin if full awareness and knowledge of the sin is present but chosen anyway.
The Catholic Church is not against all birth control, only artificial birth control. For example, natural family planning uses a natural birth control method which the Church allows.
I would say that if anyone, Catholic or not, is engaging in any sex they should be open to life. That is they should be open to the possibility of procreation. If they are not then they should not be engaging in an act where the natural end is procreation. It’s as simple as that.
Now, a married couple could be in a state of life where they can not have another child for financial or other valid reasons. In such a case exercising prudence in sexual relations is necessary. The natural family planning method is not sinful because it does not block the natural end of sex. It does this by practising abstinence during the woman’s fertile period. And abstinence is not sinful. Furthermore, when they do have sex during the infertile period they are still open to the possibility of procreation since they are not using contraceptives. Thus, they are not blocking the natural end of sex. And there is a distinction in not being able to have more children, but still being open to the theoretical possibility in the sexual act. Whereas, to engage in sex with contraceptives means to be not open to the possibility of procreation, because one is blocking its natural end through artificial means. The end does not justify the means.