Well let’s take the example of murder first: virtually everyone agrees that murder is objectively wrong, even if some individual considers it right. An individual would still be jailed for murder whether he thought it was okay or not (though in some cases they might put you in a mental asylum etc. etc.)
It’s the same with abortion. The Catholic Church teaches that abortion is objectively wrong, as it is the destruction of an innocent life. Therefore it’s perfectly reasonable to ban abortion - to protect the baby. Yes some women will try to have abortions anyway and possibly injure themselves, but when this happen it’s totally their choice: we’re not allowing them to do it, they do it anyway, and get hurt.
In the case of homosexual marriage things become more complicated. I suppose one line of arguing is that one gay person is harming another by marrying him/her because it is furthering their disordered lifestyle, causing them to descend further into sin. There are also various sociological arguments against gay marriage, which would probably work better if the person is atheistic/agnostic but it’d take too long to get into them here (Catholic Answers has quite a bit of data on that, so you can find it there).
It could be asked how the teachings of the Church accurately reflect our consciences, and that is, of course, opening up a 'nother can of worms. You can either leave the Church out of it entirely and argue in both cases based on the morality common to all humanity, or, if the person is already a Christian, you can try to defend the sanctity of the Catholic Church theologically. Neither one of those options is very easy but I guess this sort of stuff never is
Good luck, hope I was of some help!