So there is no point in discussing the subject.
Christian sin is a concept relating to the reality of God and his plan of salvation. Normal, mainstream, secular people deny this framework; hence they equate sin with illegal, and assume that sin implies some kind of physical punishment.
Such a misunderstanding is inevitable - if you don't believe that God is real, then naturally all morality must be secular, hence ultimately legalistic.
But it is an easy trap to engage in talking at cross purposes. Therefore, if a secular person asks 'do you/ Christians regard X as a sin?' - then you need to ask-back: 'What do you mean by 'sin' and what are its implications for you?'
Only then can you give an answer. There is no point in saying 'Yes! I/ we regard X as a sin' if that statement is absolutely certain to be misunderstood.
The only possibility for going further than such a denial is when there is a genuine interest from the other party in understanding what is meant by sin; and such a conversation would take at least several minutes of exposition and clarification. Most people can't be bothered; indeed they prefer to misunderstand.
The interesting thing is that - by the above test - 'liberal Christians' (e.g. the type who make a point of advertising the Diversity of their churches, or who make a point of apologising for past bad behaviour .. not by themselves, but of their predecessors); such types also fail to understand the nature of sin - in this respect they are just as ignorant as mainstream atheists; however much they dress-up their ignorance in Christian terminology.
(Which should, itself, be enough to remove any lingering doubts you may have gullibly entertained regarding their Christian falseness and fakery.)