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If you do give a reason that they tell you it's not good enough to base a preference on, remind them that it may not be good enough of a reason for them to develop a generalized preference, but it is good enough for you to.

Your mind is yours, they have as much a right to judge your mentality as you have to judge theirs, your life is yours, and you have as much right to live it your way as they do theirs.

Bring it back to the principle of non-aggression or the principle of no-harm, so long as you are not infringing upon the ownership rights of others to their (physical) person and (physical) property, you are respecting the freedoms of others, and are thus acting within the boundaries of your own freedoms.

They want to drag you into a moral debate over what is good, do not let them, simply restrain the conversation to how what you are doing is not sufficient to be labelled evil, and continually remind them that morality is relativistic and they are not any more fit to judge you than you are them, the only judge which could be considered impartial is the consequences of you going your way and them going theirs (in others words, you and they will fight about it and the best man will win).

Matters of morality cannot be settled by any means but by seeing which one outlasts the other. Ethics are traditions, ways of thinking and doing which are only able to be judged by their results. Traditions form by the Darwinistic process of natural selection is applied to memetics, when the people who do things one way outlast the competing alternatives, traditions are established. Traditional folklore, for example, tends to be the stories that were better at passing themselves on, and the people who told them were better at continuing their bloodlines.

So the solution to a moral debate is often whose beliefs can outlast the other, this applies in two tiers, both how well the meme can spread itself through the individuals, and how well the individuals can multiply themselves while still passing on their traditional way of thinking to their successors.