You decided to have sex, you know sex can result in pregnancy, the child is half yours.
Why is it some guys think they have the option to opt out of taking responsibility for a child they helped make, even though opting out of responsibility in any other area in life is considered a very bad thing?
You agreed when you inserted your penis into her vagina. If a baby would be such a burden on your life and is something that you just do not at all want, then don't have sex because that is how they are made. It's no different than someone who knows they become violent when they get drunk, so they refrain from drinking even though they may not want to, but it's better than another assault and battery charge.
Why should men not have to "man up" and accept responsibility of their actions? Yes the woman is the one that has to carry to term, but it takes two to make a baby. It's like a two-man demolition crew causing a botched demolition that caused damage to nearby buildings, and one of them tries to dodge the consequences because it was the other guy that pushed the button even though both of them wired the explosives.
Or if you get drunk, drive home, hit someone, and try to get out of the consequences because that person you hit shouldn't have been at that particular place and particular time.
Sex is a powerful force, almost coercive in its effects on the actions of people, men especially. There is scope to pursue sexual relations primarily and solely for the pleasure involved. I would think that such an engagement if employing reasonable measures of contraception implies a desire and agreement not to create a baby, perhaps enough so to support a participant’s defence that they never agreed upon raising and supporting a baby.
It’s one thing for the male partner to decide upon himself to support thereafter should the female decide to continue the pregnancy, but to enforce it, there is definitely some degree of coercion, even if it leads to better outcomes for children in some cost-benefit type analysis. Enforced financial support is not a completely innocent procedure imo.
When it comes to protected sex, the 'contract' between the 2 people is for the sex, not procreation. Now of course that includes the small risk of accidental pregnancy, but that would be an incidental outcome, rather than a principle one. The resultant pregnancy is by chance resulted from the action rather than the action being performed with the specific aims of pregnancy. I think that’s a real difference, and one that does seriously impact responsibility. In deciding to keep the child after clear efforts to avoid the pregnancy is to change the terms somewhat, if not radically.
Additionally, one could argue that by employing contraception, one assumes that if there was a failure or malfunction, further measures would be taken to blockade the progression, at least in a society where there’s no specific circumstances to suggest otherwise that could reasonably be anticipated by the individual.
Of course I’m only jumping in here with some arguments to bolster the other side of this thing. What i think any one person 'should' do is another question, and one that can only be meaningfully discussed on a case by case basis, as context can be of paramount importance.