I've been thinking more about what I would and would not feel comfortable with, really from anyone other than my wife rather than only from another man.
I think I might be OK with coaching over the phone or Skype (audio I think), probably either way but I was originally thinking in terms of having someone talk me through one of the more powerful enemas to see how the experience was different. I could return the favor here, but it probably wouldn't be very sexual either way.
The other thing that might tempt me would be someone who really knew what they were doing showing me how to use a colon tube. The focus here would be on learning the skill with no genital contact, so I don't know if that would count as play or not. It could go either way (M or F), but I imagine that there are probably more M's willing than F's, but it would need to be someone I could really trust either way that it wouldn't go any further and would be safe.
In all of these there would be two conflicting pulls - on one hand having a woman who was not my wife do it might open up the whole sex thing, which would be uncomfortable. On the other hand doing stuff with a guy is doing stuff with a guy, but then again I've already had guys do things to my ass that were non-sexual, so that might be less of an issue as long as everyone kept the sex part off the table. Best of course would be to have my wife do it, and I'm not sure if I'd "pull the trigger" on anyone else.
General play "for fun", fisting, prostate massage, other sex stuff is a no. I don't think I would ever be comfortable with that.
In any case I don't think it would really ever happen.