Prolly not the answer you're wanting, but 2 male stars widely assumed to have had sex together in reality (and who actually lived together for years) were Cary Grant and Randolph Scott. And thinking about them being together in bed is easy, since they made a handsome couple, IMHO. I find the photos of them lounging around their Malibu beach house in short bathing trunks rather homoerotic for the 1930s, no women to be seen, just the 2 of them looking very cozy & domestic.
Two others less certain to have been lovers, and then only occasionally, but whom I like to imagine could have been, are Errol Flynn and Tyrone Power. They were the most beautiful male screen stars of their day.
And interestingly, I've found that other gay guys my age, who like me saw them on TV when their old movies were played a lot, all had juvenile crushes on them. I knew I did, even before I realized what gay was.
Only a few years ago I realized I never had a crush on a female movie star when I was growing up, only for men like Flynn, Power, and Grant. I know, big DUH, but still, I'm always amazed to realize how little I once knew about myself.
As for modern male celebs having sex, none come to mind, since none come to mind for any purpose. Well, maybe Brad Pitt, who seems like a decent guy. I find them 2-dimensional & boring, most of them scruffy creatures off-camera or off-stage, and thoughts about them rarely come to me, certainly no fantasies.