Your response seemed to suggest that believing the creed is all that is necessary to be a Christian - independent of your lifestyle choices / sexual activity.

I don't know why it would suggest that to you. The OP asked why the page says "we believe this is sinful" if that's just one opinion and other Christians might disagree.

You responded by basically saying anyone who disagrees is not really a Christian. To which I responded that opinions on how we should understand God's expectations on sex have never been considered fundamental beliefs that determine whether someone is Christian or not. We can disagree on God's expectations on sex and still be Christians who take our faith and the scriptures seriously.

the need live in a way that seeks to reject all sin

This was never in question. The question was in regards to the statement "we believe that such a sexual relationship is sinful" which is your own understanding of what God's expectations are. That is not a universally held belief and not something that any of the ecumenical councils decided. It's not a marker for whether one is a Christian at all and merely reflects your own understanding of scriptures.

My point is that somewhere along the line sexual activity becomes a sin; the only question is where. Do you agree?

I believe certain sexual activities are sinful. But I don't see how this would be some kind of continuum, so I'm not sure how the statement "somewhere along the line" makes any sense.

And would you agree a permanent relationship - 'marriage' is the only place for sexual activity for a Christian.

No

We can then debate whether marriage must be between a man and a woman!

I'm not really interested in debating that. I was only interested in pointing out the fact that people can disagree with you on this issue and still be legitimately considered Christian.

There are a variety of opinions on sexuality within the Christian community. Whether you like it or not, there are God-fearing, spirit-filled Christians who disagree with you on this issue.