Some interesting thoughts here from Henry Cate, but I am not quite convinced by the absoluteness of his last paragraph. I'd say that there are passive men whose sexual satisfaction and arousal are simply not experienced in their penises. I have met guys who are gagging to be f*cked (I'm not speaking of money boys or commercial transactions) who achieved no erections, or only very weak ones. Their own erections did not seem to be a matter of any interest to them; and, indeed, to take an interest in their having an erection could prove an embarrassment to them - they can have a feeling of failure. Personally I find it reassuring if a passive partner gets an erection - I must be doing something right! - and it makes the experience feel more 'mutual'.
In general, of course, I'd agree that a reactive erection tells us something about a partner's involvement, but there are so many variants of sexual behaviour and response that I don't always put it down to a fault in myself that a partner doesn't get an erection. The ideal for me is a passive partner who cums during sex without touching himself - do they call it 'prostate ejaculation'? I'm looking forward to the next time it happens.