I've actually known a few couples similar to what you describe and they too seem very happy. No way I could do that though... yuck!
It might be helpful to you to think about why your response is "yuck!" Using a strap-on would really be at the far edge of my sexual comfort zone because I'm not quite comfortable enough in my femininity to do that and also because I simply don't like toys or paraphernalia very much. So, in order to do that I would need some combination of alcohol, feeling good enough about my femininity to "act" the masculine, a partner with whom I was intimate enough to know his boundaries/desires, a partner from whom I could "borrow" validation of my femininity, feeling good enough about sexuality in general to perform the edgy and a high level of general arousal/ horniness. I have gone for the prostate on a couple men and I got the same response from both of them which was along the lines of "I must admit I really liked it but it was a bit over my boundary line so it makes me psychologically uncomfortable." I have no real urge in my sexuality to push a man into that level of vulnerability. On that edge of things, I just want a guy who is comfortable enough in his masculinity to allow me to maybe grab his *ss and f*ck him from below. It's his choice if he wants to see that as high monkey or low lioness. I'm good either way.
"Tell me, what is it you plan to do with your one wild and precious life?" - Mary Oliver