Ok, it's NOT that I'm gay, but that guy looks...

The man in the OP might have been doing some damage control, IMO, explaining a little too much-about how there should be no doubt that he was not considered gay.

Maybe, the guy was deep in the closet, and noticed a completely different great looking man.
Then was forced to make the quick save, by referring instead to “…that’s one ugly man!”,
after digging himself into a hole, with the comment : “Look, I’m not gay, but…”

A close friend of mine, who is openly gay; enjoys the site of an attractive man,
and freely shares his opinion with friends when we are together.

Nobody here is saying anything of the sort. Obviously I can easily tell that a very fit guy and an extremely obese guy are not identically attractive. The issue is when you have, say, two guys who are broadly similar in build, health etc.

Well if two men are broadly similar in looks, chances are that someone who IS into guys wouldn’t have much of a view about which one is more attractive (although I stand by the caveat in my previous post).

There is also the fact that we have our own idiosyncratic interests when it comes to attraction, so those are going to propel us towards different kinds of people and so again the whole idea of there being a right or wrong answer when it comes to is someone attractive doesn’t really work. To give an example, I think this man is sex on legs, but I’ve yet to meet another person alive who agrees with me. Doesn’t stop him being attractive to me.