If a man and a woman have consensual sex, and they use contraception, and they both agree that they do not want a child, then an early term abortion is the only sensible recourse in the case of contraception failure.
In the above situation, one partner turning around and saying "Actually... I want to keep it" is breaking faith with the previously made agreement on how contraceptive failure would be dealt with, and the other partner is entirely within their rights to walk away. The consent to engage in sex in the first place was predicated on the understanding that a child would not be the result.
When it comes to one night stands, rather than long term relationships, the implicit agreement is the same - that a contraception failure will be resolved with the morning after pill or a very early term abortion. Nobody enters into a one night stand with the intention of creating a child.
If a woman or man has a strong moral objection to an early term abortion, they should make that absolutely clear to any sexual partner before they have sex. Flat. That way the partner can make an informed choice about risk, including whether they want to double/triple their contraceptive measures or just refuse to have sex in the first place.
Any child that is born should be the result of two parents who are happy to conceive together, in order to maximise their life chances.
If one parent does not consent to conception, then that parent has no obligation to raise that child, provided that their partner knew and understood that this was their position. A woman who knows her partner does not want a child should have no expectation that he will be a father to it. A man who knows his partner will not have an abortion should refrain from having sex with her, or he should get a vasectomy.
If you haven't spoken about it, then don't have sex at all in the first place.