Time to introduce some variety back into your sex life?
If we're interupted then sometimes the moment just passes. And we may not be interested at the start. But I haven't yet lost interest part-way through.... I now have to find the other thread to see if this is something which is eventually likely to happen!
Spidermama I can only assume you have already orgasmed by this stage so that is why you're loosing interest? In that case I think it is perfectly fine to just let your mind wonder while they 'finish' the job.
Usually if I have already finished DH just gets on with it...he can get there pretty quickly! If I 'lose interest' ie I know I'm not going to get there I turn over (pre-preg days) and he finishes in seconds... There have been times when I was really not in the mood and have stopped it, but that's usually before penetration. DH has never pressured me once I have made it clear it's not happening, although he will try to carry on groping me if I'm not clear!