As I said, the point at which it become excessive can be subjective, but it's a real theoretical possibility. You can't deny the possibility of venial sin simply because it's difficult to determine when it happens. There are a lot of sins that progress along a relatively-vague continuum ... including gluttony.
Is one piece of cake gluttony? Unlikely. Are two pieces? Maybe. Three? Most likely. Ten? Undoubtedly. There's no hard fast rule (i.e "clear metric") as to how much cake you have to eat before it becomes a sin. Or how fast you eat it. So, even with eating, there's a certain amount of subjectivity to it. So much for the "clear metrics". When you get to two pounds of cake, then there's no longer any doubt. Similarly, with sex, you usually know when you crossed that line.
The problem is, no one ever gets specific about what constitutes a discrete entity -- "a piece of cake" -- in the sex world. Is it "minutes of being together"? Is it "# of body parts involved"? or what?
In the case of gluttony, you know you've crossed the line because you feel sluggish, have a stomachache, feel sick, etc. afterward. But in the case of sex, what if you're not excluding the primary motive (begetting of children)? How do you know when "you've gone too far"?
There sure isn't much information, considering how important a topic this is. It affects most laymen, since the majority of mankind is (was, or will be) married.
The ambiguity and lack of information is PRECISELY what allows heretics like "Heitanen" to step in and preach Puritan values in this matter. And I'm sure there are others on the other side wishing to preach their liberal stance on the matter. Why can't there be guidelines, somewhere, in a moral theology manual or something? There are a lot of Cathlolics of good will wanting to get this right.
And don't say, "Oh, you know...you just know" because in today's world, you can't "know" anything just by osmosis. The world is so messed up and sex-crazed, we can't take anything for granted. Culture is no help today.