some are turned on by knowing you've had an other guy.
it'll only make him even more horny… it's crazy but that's how men function!
the more you let loose, the more he'll crawl!

What he said. This always works. Gay or Straight.

What I'm saying is it's probably not really you.
Your partner may think he's "had" you and is bored.
Maybe introduce something different.

You could try role play, to simulate being someone new.
Or you could try to introduce something you haven't tried before, like toys.
Or for some people, it's a setting. So, maybe try sex in different places, like go on a trip, or rent a hotel room.
Maybe suggest trying something he hasn't tried before, or asking if there's something he's wanted to try but hasn't.

Interest is different for everyone. All you can do is try to figure out how to keep it interesting.
It would be nice to say, "just do this thing" and everything will work out, but life doesn't work that way.

It may be that his fetish, is "new" as it is with many guys, and there may be nothing you can do about it.

Sometimes it's a matter of not feeling appreciated.