While nearly all moral systems have some shared premises, there are differences Anthropologists have found that all societies come up with essentially the same codes of ethics, regardless of religious beliefs or lack of religious beliefs.
For example, you can count on finding a prohibition against murder. However details will differ on when killing of other people is permissible, and therefore not murder.
In areas of sexual activity, all bets are off. I sometimes speculate that this is a matter of whether the act under scrutiny includes victimization. For example, rape is generally prohibited, but consensual sexual activity can range from deserving the death penalty (arabic) to the opposite end of the spectrum, and being encouraged even among strangers (pacific). My theory is that in these cases, the matter comes down to the purpose or essence (if you will) of the act. In a pacific islander’s culture, the purpose or nature of sex is not defined as procreative, and therefore non procreative sex is actually seen as a moral good, as it promotes happiness and intimacy. In the Church, the pro-creative impetus is the postulate from which other standards derive, and which constrains the point of view. Therefore, even without the scriptural prohibitions to homosexuality (which are not always interpreted in the same way), the Church is opposed to non-procreative sex on general principle for all sexual activity.
So, we get into this ground of moral relativism… where a devout Catholic may say… I am right and your disagreement does not change that “fact”. But unless the other party accepts that Catholic system of principles and postulates as being universally true, then the entire argument fails, at least to the extent that the other person accepts your “truth”.