Of course it's natural for us straight guys to think and talk about size. We have dicks too. And it's not secret that a major preoccupation for many straight guys is how women will react to their sizes. Look at all the "add inches" spam you get in your e-mail--much of it is designed to tell men that women will find them irresistible if only they add the 2-3 inches promised.
I do think there are ages/situations when it is much more naturally to come up than others. Most size-related conversations I have had with fellow straight guys were back in my college days, a time when we all were just recently out of puberty and our penis-growth stage of life and lived in dorms where we showered together. Only natural that it should be a topic of interest in that situation. Now, in my adult life, the topic comes up occasionally, but less often and in less candid terms.
BigA's comment about demonstrating a jo method under a blanket reminds me of demonstrations I put on in college. My buds in the dorm knew I was hung from seeing me in the showers and word of mouth. I knew many of them were curious about seeing it hard but would never dare admit it. So, a couple of times, I demonstrated my 11" erection making a tent under my blanket. At first the guys would accuse me of using my knee or something, till I would stick my legs and arms fully outside the covers and dare them to whip back the blanket if they still doubted. They never did, and seemed duly impressed with the tent. :smile: