If you're starting with complex axioms like

> "sex should only be allowed between partners in long term monandrous relationships"

then what you've got is a moral justification, not an ethical one.

I've only dabbled in moral philosophy (the study of ethics!) but my understanding is that the more common use amongst professional philosophers is that ethics relates to a system established usually by a particular group, like a legal code or religious doctrine whilst morals concern an individuals actions. Morality then is considered to concern a naturally occurring right whilst ethics concerns a constructed set of rules (often based on or intended to enforce particular morals).

The terms in common parlance are used interchangeably and unfortunately even those (possibly novices) working in the art use the terms in a contrary way - http://www.philosophyblog.com.au/ethics-vs-morality-the-dist... highlights the mess very well.

Perhaps you could enlarge on your objection, is it merely semantic or does it have a substantial element too?