I wanted to keep the focus on the abortion issue, so I have said very little about some of the background arguments relating to each of these issues. Here are a few classic thought experiments to show the problems with happiness based ethics aka Utilitarianism.

Ursula Le Guin and Dostoyevsky both posited societies that had created perfect happiness that depended on the torture or slaughter of one innocent child. The result would be millions of happy people and one innocent miserable child writhing in agony. That's a net win according to Utilitarianism, but this is wrong because the child does not deserve to suffer. Or suppose a city is filled with rioting because the rioters want a certain murderer prosecuted. The police could stop the riot, by framing an innocent man for the murder and hanging him. The only person who loses because of this is the hanged man, so again a net win from the utilitarian position, but wrong because the hanged man is innocent.

Other examples: A television show watched by millions of people in which an innocent person is tortured. By what Utilitarians call the Hedonic Calculus, the enjoyment of the millions of people watching the torture cancels out the suffering of the one person being tortured.

If ten percent of a society is enslaved, and this results in more happiness for the remaining 90%, then slavery is OK.

Utilitarianism also requires continuous population growth, because by the Hedonic Calculus two million happy people is worth more than 1 million happy people. Of course real population growth trashes the planet, which leads to less happiness in the long run. But even if it didn't, it's absurd to think we have an obligation to increase the population if all of the new people are happy.

There are more examples, but I think you get the idea.