GalaGirl you are very perceptive and your questions have cut through the confusion to the root of the issue. Thank you.
Yes, he has Aspergers and to some extent it can create issues with communication. That is why I was so pleased when he expressed what he wanted clearly and with a degree of consideration, and that is why a part of me wanted to say yes to his request. It seems harsh to say no when he has tried hard to get it right.
I have asked him the questions that you raised. He is not worried about top space exactly, part of the reason he wants me there is because he is afraid of being too unfeeling and not being able to deal with what happens afterwards (sub drop etc). He is also worried about the new woman (J) being too needy/clingy. If he has picked up on that then I really don't think he should get involved - but of course that is his decision not mine. She is capable of knowing what she is getting into and of looking after herself, and if he wants this he needs to cope with all that it entails (it would just be easier for him to rely on me).
He said that having me there would make it more exciting for him and I think he is being honest with me about that. He thinks I would enjoy it and I can see why he thinks that, I have been a bit reluctant about other things in the past and have got used to them and actively enjoyed them after a while. However I am quite clear about this, it wouldn't hurt me or upset me, but I just don't want it.
So what you were hearing was me arguing with myself, wanting to say no but feeling 'obliged' to say yes (partly because he asked nicely and partly because it is easier to just be there as an 'enabler' than to help him to pick up the pieces afterwards.
I have decided to say no about being there, however if C agrees to it happening I will help him to plan it (if he wants my help) and talk it through with him so that he feels as safe and confident as possible.