- Yes, fornication is a mortal sin.
- Contraception adds an additional mortal sin, even when used outside of marriage. It is possible for two mortal sins to be “less evil” than one mortal if the one mortal sin has sufficiently severe consequences. Note that unintended pregnancies are bad consequences, but are not grave consequences (life and death, starvation, adultery, etc).
What is “less evil,” fornication without contraception or fornication with contraception? The answer is black and white with (1) direct contraception via the Crime of Onan and (2) no life threatening disease transmission. Then given the malice of Onan is morally equivalent to a man attacking his wife, therefore, fornication without a condom (which has the risk of pregnancy) is undoubtedly “less evil” than fornication with a condom.
Changing the hypothetical to HIV transmission, Pope Benedict XVI thinks that condom use is never a real or moral solution.
Using a condom means committing two intrinsically evil sins, not just one. However, while Pope Benedict XVI does think that there is an extra sin with a condom, he said it may be a step in the right direction, “less evil,” to use one than to not use one. (Less evil does not mean a lesser of two evils, which only applies to situations where the act is not intrinsically evil).
The answer of whether condoms are “less evil” with potential HIV transmission depends upon whether the positive of reducing the risk of HIV offsets the negative malice that comes from the moral object, which is so great that it makes the act morally equivalent of an attack. In Africa, where HIV can be a death sentence, condoms are likely “less evil”: reducing the risk of death is a greater positive than the negative of the means to achieve that, which is morally equivalent to an attack.
But although it is “less evil,” it is still evil. (It is not a “lesser evil,” in which case the act is in itself moral and potentially justifiable for a grave reason). Intrinsically evil acts are never justified by consequence or intention!
If technology advances to the point that it can substantially and affordably improve HIV symptoms, then the grave consequences of HIV transmission will no longer have the same gravity that it currently does. Therefore, fornication with a condom would be a greater evil, since the consequences of an unintended pregnancy, while bad, are not grave (starvation, losing your house, adultery, severe harm to children) required to offset the moral equivalent of an attack (from the moral object of condom use).
Does the same analysis that applies to Onan apply to other forms of contraception? This requires more Magisterial clarification. For an act that is not contraception, but just avoiding pregnancy, the most probable opinion is that serious reasons are sufficient to morally practice NFP (and not grave), However, the totally different hypothetical, fornication with or without direct contraception, could be very different. It might be true, it might not be true, that the malice of direct hormonal contraception combined with fornication only requires serious (and not grave) offsetting circumstances to be “less evil.”
