Thanks for the replies. It isn't hypothetical - basically I have a really good male friend I've known for six years and we trust each other completely. He has always been attracted to me but I've seen him as more like a brother. We've both just come out of failed relationships and were innocently discussing missing the 'physcial side' when he made the proposal. On paper it makes sense - we both have major health problems making a full commitment tough at the moment, and we are both very lonely and missing physcial contact. We like eachother although for me it's more platonic. I was straight with him that if I found a long term partner our arrangement would have to end, and he is absolutely fine with that. I trust him because I've known him a long time and believe he would stop whenever I wanted him to. My only concern is his feelings for me and whether he's secretly hoping more will come of it. I've been absolutely clear from my point of view that we would simply be good friends with sex though.

I think there's a sort of element of 'I shouldn't do this' that makes me want to. I've always been a quiet person and settled down into marriage and a child very young.