The simple answer is, "just have sex". Honestly, I've never had a partner care about my ostomy appliance. Sometimes I put on a mini closed end pouch (see Convatec - 401529 as an example). I usually do empty it before hand, no matter what I'm wearing.
I'm athletic and I'm fairly active (snowboarding, motorcycle racing, hiking, tango dancing, deep sea diving, cycling, yoga) and I've never found the need for a retention belt of any kind. If it's a "new partner", I just focus on them more than my ostomy. No need to go into all the details before sex, save that for the pillow talk afterwards. My typical response to "So what is that?" is, "I had a surgery when I was a kid (30 years ago), there isn't anything I can't do with it there". If they seem even more curious I just say, "if you want to know more, just ask".
Be confident, nobody has a perfect body, just ask Arnold Schwarzenegger when he was in his prime and he'll say he didn't like his calves. LOL
Confidence is sexy, insecurity... not so much. I have yet to be with a woman who minded and if they did, there's more problems with them besides their shallowness.
The biggest challenge is just getting over your own mental blocks and self-image. I realized years ago that I just didn't care anymore when I got into the hot springs naked with everyone else. In that situation you find out that everyone is the same, with or without a bag attached.
Best of luck....
ps. The stealth belt looks pretty cool, I can certainly see the application.