I think it's either Sheen (which won't come as a surprise to anyone really) or RDJ. I think RDJ is more "mainstream" and considered an action star due to the Ironman franchise, so has more to lose.

There have been rumors about both of them being HIV positive for years though.

Just to toss a few more randos into the ring with questionable sexuality - Hugh Jackman, Jake Gyllenhaal, Jeremy Renner.